View source for Ctenophora

{{Short description|Phylum of gelatinous marine animals}}
{{hatnote group|
{{For|the genus of crane flies|Ctenophora (fly)}}
{{distinguish|Ctenophorus}}
}}
{{use dmy dates|date=October 2024}}{{Good article}}
{{Automatic taxobox
| name=Comb jellies
| fossil_range=[[Cambrian Stage 3]]–[[Holocene|Present]], {{fossil range|518|0|earliest= 600
| ref=<ref name=Chen2007>{{cite journal | last1=Chen | first1=Jun-Yuan | last2=Schopf | first2=J. William | last3=Bottjer | first3=David J. | last4=Zhang | first4=Chen-Yu | last5=Kudryavtsev | first5=Anatoliy B. | last6=Tripathi | first6=Abhishek B. | last7=Wang | first7=Xiu-Qiang | last8=Yang | first8=Yong-Hua | last9=Gao | first9=Xiang | last10=Yang | first10=Ying | display-authors=6 | date=April 2007 | title=Raman spectra of a Lower Cambrian ctenophore embryo from southwestern Shaanxi, China | journal=[[Proceedings of the National Academy of Sciences of the United States of America]] | volume=104 | issue=15 | pages=6289–6292 | doi=10.1073/pnas.0701246104 | doi-access=free | pmc=1847456 | pmid=17404242 | bibcode=2007PNAS..104.6289C}}</ref><ref name=Stanley1983>{{cite journal | last1=Stanley | first1=G.D. | last2=Stürmer | first2=W. | title=The first fossil ctenophore from the Lower Devonian of West Germany | journal=[[Nature (journal)|Nature]] | volume=303 | issue=5917 | pages=518–520 | date=9 June 1983 | doi=10.1038/303518a0 | s2cid=4259485 | bibcode=1983Natur.303..518S}}</ref><ref name=Morris1996>{{cite journal | last1=Conway Morris | first1=S. |author1-link=Simon Conway Morris | last2=Collins | first2=D.H. | doi=10.1098/rstb.1996.0024 | title=Middle Cambrian Ctenophores from the Stephen Formation, British Columbia, Canada | journal=Philosophical Transactions of the Royal Society B: Biological Sciences | volume=351 | issue=1337 | pages=279–308 | date=29 March 1996 | bibcode=1996RSPTB.351..279C}}</ref>}} (Possible [[Ediacaran]] record<ref name=Tang2011/>)
| image=Comb jelly.jpg
| image_caption=
| taxon=Ctenophora
| authority=[[Johann Friedrich von Eschscholtz|Eschscholtz]], 1829
| subdivision_ranks=Classes
| subdivision=*[[Tentaculata]]
*[[Nuda]]
*{{extinct}}[[Dinomischus|Dinomischidae]]<ref>{{Cite journal |last1=Zhao |first1=Yang |last2=Hou |first2=Xian-guang |last3=Cong |first3=Pei-yun |date=2023-01-01 |title=Tentacular nature of the 'column' of the Cambrian diploblastic Xianguangia sinica |url=https://www.zora.uzh.ch/id/eprint/234398/9/Tentacular_nature_of_the_column_of_the_Cambrian_diploblastic_Xianguangia_sinica.pdf |journal=Journal of Systematic Palaeontology |language=en |volume=21 |issue=1 |article-number=2215787 |doi=10.1080/14772019.2023.2215787 |bibcode=2023JSPal..2115787Z |issn=1477-2019|doi-access=free }}</ref>
*{{extinct}}[[Scleroctenophora]]<ref name=Shu-Zhang-etal-2015>{{cite journal |last1=Shu |first1=Degan |last2=Zhang |first2=Zhifei |last3=Zhang |first3=Fang |last4=Sun |first4=Ge |last5=Han |first5=Jian |last6=Xiao |first6=Shuhai |last7=Ou |first7=Qiang |date=July 2015 |title=A vanished history of skeletonization in Cambrian comb jellies |journal=[[Science Advances]] |volume=1 |issue=6 |article-number=e1500092 |doi=10.1126/sciadv.1500092 |pmid=26601209 |bibcode=2015SciA....1E0092O |pmc=4646772}}</ref>
}}
'''Ctenophora''' ({{IPAc-en|t|ə|ˈ|n|ɒ|f|ər|ə}}; {{singular}}: '''ctenophore''' {{IPAc-en|ˈ|t|ɛ|n|ə|f|ɔr|,_|ˈ|t|iː|n|ə|-}} {{etymology|grc|''{{math|κτείς}} (kteis)'' |comb||''{{math|φέρω}} (pherō)'' |to carry}})<ref name="Fowler 1911">{{cite EB1911 |first=George Herbert |last=Fowler |wstitle=Ctenophora |volume=7 |page=593}}</ref> is a [[phylum]] of [[marine invertebrates]], commonly known as '''comb jellies''', that [[marine habitats|inhabit sea waters]] worldwide. They are notable for the groups of [[cilia]] they use for swimming (commonly referred to as "combs"), and they are the largest animals to swim with the help of cilia.

Depending on the species, adult ctenophores range from a few [[millimeter]]s to {{cvt|1.5|m|ft|sigfig=1}} in size. 186&nbsp;living species are recognised.<ref name=WoRMS>{{cite report | date=2019-09-18 | title=World Register of Marine Species | publisher=VLIZ | type=database | url=http://www.marinespecies.org/imis.php?dasid=1447&doiid=170 | access-date=19 February 2024}}</ref>

Their bodies consist of a [[mesoglea|mass of jelly]], with a layer two cells thick on the outside, and another lining the internal cavity. The phylum has a wide range of body forms, including the egg-shaped [[Cydippida|cydippids]] with a pair of retractable tentacles that capture prey, the flat, generally combless [[Platyctenida|platyctenids]], and the large-mouthed [[Beroidae|beroids]], which prey on other ctenophores.

Almost all ctenophores function as [[predator]]s, taking prey ranging from microscopic [[larva]]e and [[rotifer]]s to the adults of small [[crustacea]]ns; the exceptions are juveniles of two species, which live as parasites on the [[salp]]s on which adults of their species feed.

Despite their soft, gelatinous bodies, fossils thought to represent ctenophores appear in [[Lagerstätten]] (well-preserved fossil beds) dating as far back as the early [[Cambrian]], about 525 million years ago. The position of the ctenophores in the [[tree of life (biology)|"tree of life"]] has long been debated in molecular phylogenetics studies. Biologists proposed that ctenophores constitute the second-earliest branching animal lineage, with [[sponge]]s being the sister-group to all other multicellular animals (''Porifera sister hypothesis'').<ref name=Simion2017>{{cite journal |last1=Simion |first1=Paul |last2=Philippe |first2=Hervé |last3=Baurain |first3=Denis |last4=Jager |first4=Muriel |last5=Richter |first5=Daniel J. |last6=diFranco |first6=Arnaud |last7=Roure |first7=Béatrice |last8=Satoh |first8=Nori |last9=Quéinnec |first9=Éric |last10=Ereskovsky |first10=Alexander |last11=Lapébie |first11=Pascal |last12=Corre |first12=Erwan |last13=Delsuc |first13=Frédéric |last14=King |first14=Nicole |last15=Wörheide |first15=Gert |last16=Manuel |first16=Michaël |display-authors=6 |year=2017 |title=A large and consistent phylogenomic dataset supports sponges as the sister group to all other animals |journal=[[Current Biology]] |volume=27 |issue=7 |pages=958–967 |doi=10.1016/j.cub.2017.02.031 |doi-access=free |bibcode=2017CBio...27..958S |pmid=28318975}}</ref> Other biologists contend that ctenophores diverged earlier than sponges (''Ctenophora sister hypothesis''), which themselves appeared before the split between [[cnidaria]]ns and [[bilateria]]ns.<ref name=Dunn2008>{{cite journal |last1=Dunn |first1=Casey W. |last2=Hejnol |first2=Andreas |last3=Matus |first3=David Q. |last4=Pang |first4=Kevin |last5=Browne |first5=William E. |last6=Smith |first6=Stephen A. |last7=Seaver |first7=Elaine |last8=Rouse |first8=Greg W. |last9=Obst |first9=Matthias |last10=Edgecombe |first10=Gregory D. |last11=Sørensen |first11=Martin V. |last12=Haddock |first12=Steven H.D. |last13=Schmidt-Rhaesa |first13=Andreas |last14=Okusu |first14=Akiko |last15=Kristensen |first15=Reinhardt Møbjerg |last16=Wheeler |first16=Ward C. |last17=Martindale |first17=Mark Q. |last18=Giribet |first18=Gonzalo |display-authors=6 |year=2008 |title=Broad phylogenomic sampling improves resolution of the animal tree of life |journal=[[Nature (journal)|Nature]] |volume=452 |issue=7188 |pages=745–749 |doi=10.1038/nature06614 |pmid=18322464 |s2cid=4397099 |bibcode=2008Natur.452..745D}}</ref><ref>{{cite journal |last1=Baxevanis |first1=Andreas D. |last2=Martindale |first2=Mark Q. |last3=Mullikin |first3=James C. |last4=Wolfsberg |first4=Tyra G. |last5=Dunn |first5=Casey W. |last6=Haddock |first6=Steven H.D. |last7=Putnam |first7=Nicholas H. |last8=Smith |first8=Stephen A. |last9=Havlak |first9=Paul |last10=Francis |first10=Warren R. |last11=Koch |first11=Bernard J. |last12=Simmons |first12=David K. |last13=Moreland |first13=R. Travis |last14=Nguyen |first14=Anh-Dao |last15=Schnitzler |first15=Christine E. |last16=Pang |first16=Kevin |last17=Ryan |first17=Joseph F. |display-authors=6 |date=2013-12-13 |title=The genome of the Ctenophore ''Mnemiopsis leidyi'' and its implications for cell type evolution |journal=[[Science (journal)|Science]] |volume=342 |issue=6164 |article-number=1242592 |doi=10.1126/science.1242592 |pmid=24337300 |pmc=3920664}}</ref> Pisani ''et al''. reanalyzed the data and suggested that the computer algorithms used for analysis were misled by the presence of specific ctenophore genes that were markedly different from those of other species.<ref>{{cite journal |last1=Pisani |first1=Davide |last2=Pett |first2=Walker |last3=Dohrmann |first3=Martin |last4=Feuda |first4=Roberto |last5=Rota-Stabelli |first5=Omar |last6=Philippe |first6=Hervé |last7=Lartillot |first7=Nicolas |last8=Wörheide |first8=Gert |display-authors=6 |year=2015 |title=Genomic data do not support comb jellies as the sister group to all other animals |journal=Proceedings of the National Academy of Sciences of the USA |volume=112 |issue=50 |pages=15402–15407 |bibcode=2015PNAS..11215402P |doi=10.1073/pnas.1518127112 |doi-access=free |pmid=26621703 |pmc=4687580}}</ref> However, follow up analysis by Whelan et al. (2017)<ref name=Whelan2017>{{cite journal |last1=Whelan |first1=Nathan V. |last2=Kocot |first2=Kevin M. |last3=Moroz |first3=Tatiana P. |last4=Mukherjee |first4=Krishanu |last5=Williams |first5=Peter |last6=Paulay |first6=Gustav |last7=Moroz |first7=Leonid L. |last8=Halanych |first8=Kenneth M. |display-authors=6 |date=November 2017 |title=Ctenophore relationships and their placement as the sister group to all other animals |journal=[[Nature Ecology & Evolution]] |volume=1 |issue=11 |pages=1737–1746 |doi=10.1038/s41559-017-0331-3 |pmid=28993654 |pmc=5664179 |bibcode=2017NatEE...1.1737W}}</ref> yielded support for the 'Ctenophora sister' hypothesis; the issue remains a matter of taxonomic dispute.<ref>{{cite journal |last1=Halanych |first1=Kenneth M. |last2=Whelan |first2=Nathan V. |last3=Kocot |first3=Kevin M. |last4=Kohn |first4=Andrea B. |last5=Moroz |first5=Leonid L. |date=2016-02-09 |title=Miscues misplace sponges |journal=Proceedings of the National Academy of Sciences of the USA |volume=113 |issue=8 |pages=E946–E947 |doi=10.1073/pnas.1525332113 |doi-access=free |pmc=4776479 |pmid=26862177 |bibcode=2016PNAS..113E.946H}}</ref><ref>{{cite journal |last1=Telford |first1=Maximilian J. |last2=Moroz |first2=Leonid L. |last3=Halanych |first3=Kenneth M. |date=January 2016 |title=A sisterly dispute |journal=[[Nature (journal)|Nature]] |volume=529 |issue=7586 |pages=286–287 |doi=10.1038/529286a |doi-access=free |pmid=26791714 |s2cid=4447056}}</ref> Schultz ''et al.'' (2023) found irreversible changes in [[synteny]] in the sister of the Ctenophora, the [[Myriazoa]], consisting of the rest of the animals.<ref name=Schultz23>{{cite journal |last1=Schultz |first1=Darrin T. |last2=Haddock |first2=Steven H.D. |last3=Bredeson |first3=Jessen V. |last4=Green |first4=Richard E. |last5=Simakov |first5=Oleg |last6=Rokhsar |first6=Daniel S. |date=2023-05-17 |title=Ancient gene linkages support ctenophores as sister to other animals |journal=[[Nature (journal)|Nature]] |volume=618 |issue=7963 |pages=110–117 |doi=10.1038/s41586-023-05936-6 |pmid=37198475 |pmc=10232365 |bibcode=2023Natur.618..110S}}</ref>

[[File:Spotted Comb Jelly.webm|thumb|Spotted comb jelly]]

== Etymology ==

The New Latin<ref name="Merriam-Webster"/> name {{lang|la|Ctenophora}} is constructed {{etymology|grc|''{{math|κτείς}} (kteis)'' |comb||''{{math|φέρω}} (pherō)'' |to carry}},<ref name="Fowler 1911"/> alluding to the rows of cilia that are distinctive features of animals in the phylum.<ref name="Merriam-Webster">{{cite web |title=Ctenophora plural noun |url=https://www.merriam-webster.com/dictionary/Ctenophora |publisher=[[Merriam-Webster]] |access-date=8 May 2025}}</ref>

== Distinguishing features ==

{{Further|Sponge|Cnidaria|Bilateria}}

[[File:Pelagic ctenophores.png|thumb|upright=1.5| {{center|'''Pelagic ctenophores'''}}<br/>'''a'''&nbsp;''[[Beroe ovata]]'', '''b'''&nbsp; unidentified cydippid, '''c'''&nbsp;"Tortugas red" cydippid, '''d'''&nbsp;''[[Bathocyroe fosteri]]'', '''e'''&nbsp;''[[Mnemiopsis leidyi]]'', and '''f'''&nbsp;''[[Ocyropsis]]'' sp.<ref>{{cite journal |last1=Ryan |first1=Joseph F. |last2=Schnitzler |first2=Christine E. |last3=Tamm |first3=Sidney L. |date=December 2016 |title=Meeting report of Ctenopalooza: The first international meeting of ctenophorologists |journal=[[EvoDevo]] |volume=7 |issue=1 |page=19 |doi=10.1186/s13227-016-0057-3 |doi-access=free |s2cid=931968 |hdl=1912/8430 |hdl-access=free|pmc=4997693 }}</ref>]]

Ctenophores are distinguished from all other animals by having [[colloblast]]s, which are sticky and adhere to prey, although a few ctenophore species lack them.<ref name=Hinde2001CnidariaAndCtenophoraInAnderson>{{cite book |last=Hinde |first=R.T. |year=1998 |chapter=The Cnidaria and Ctenophora |editor-last=Anderson |editor-first=D.T. |title=Invertebrate Zoology |pages=28–57 |publisher=Oxford University Press |isbn=978-0-19-551368-4}}</ref><ref name=MillsNotesFromExpert/>

Like cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the [[mesoglea]] in cnidarians and ctenophores; more complex animals have three main cell layers and no intermediate jelly-like layer. Hence ctenophores and cnidarians have traditionally been labelled [[diploblastic]].<ref name=Hinde2001CnidariaAndCtenophoraInAnderson/><ref name=RuppertBarnes2004CnidariaGeneral>{{cite book | last1=Ruppert |first1=E.E. | last2=Fox |first2=R.S. | last3=Barnes |first3=R.D. | year=2004 | title=Invertebrate Zoology | edition=7 | publisher=Brooks / Cole | isbn=978-0-03-025982-1 | pages=[https://archive.org/details/isbn_9780030259821/page/111 111–124] | url=https://archive.org/details/isbn_9780030259821/page/111}}</ref> Both ctenophores and cnidarians have a type of [[muscle]] that, in more complex animals, arises from the [[mesoderm|middle cell layer]],<ref name=SeipelSchmid2005EvolutionOfStriatedMuscle>{{cite journal | last1=Seipel |first1=K. | last2=Schmid |first2=V. | date=June 2005 | title=Evolution of striated muscle: Jellyfish and the origin of triploblasty | journal=Developmental Biology | volume=282 | issue=1 | pages=14–26 | pmid=15936326 | doi=10.1016/j.ydbio.2005.03.032 | doi-access=free }}</ref> and as a result some text books classify ctenophores as [[triploblastic]],<ref name=RuppertBarnes2004Ctenophora/> while others still regard them as diploblastic.<ref name="Hinde2001CnidariaAndCtenophoraInAnderson"/> The comb jellies have more than 80&nbsp;different [[cell type]]s, exceeding the numbers from other groups like placozoans, sponges, cnidarians, and some deep-branching bilaterians.<ref>{{cite bioRxiv |last1=Moroz |first1=Leonid L. |last2=Norekian |first2=Tigran P. |date=16 August 2018 |title=Atlas of Neuromuscular Organization in the Ctenophore, Pleurobrachia bachei {{small|(A. Agassiz, 1860)}} |biorxiv=10.1101/385435}}</ref>

Ranging from about {{convert|1|mm|in|sp=us|sigfig=1}} to {{convert|1.5|m|ft|sp=us|sigfig=1}} in size,<ref name="RuppertBarnes2004Ctenophora"/><ref>{{cite journal | last1=Viitasalo |first1=S. | last2=Lehtiniemi |first2=M. | last3=Katajisto |first3=T. | year=2008 | title=The invasive ctenophore ''Mnemiopsis leidyi'' overwinters in high abundances in the subarctic Baltic Sea | journal=Journal of Plankton Research | volume=30 | issue=12 | pages=1431–1436 | doi=10.1093/plankt/fbn088 | doi-access=}}</ref> ctenophores are the largest non-colonial animals that use [[cilia]] as their main method of locomotion.<ref name=RuppertBarnes2004Ctenophora/> Most species have eight strips, called comb rows, that run the length of their bodies and bear comb-like bands of cilia, called "ctenes", stacked along the comb rows so that when the cilia beat, those of each comb touch the comb below.<ref name="RuppertBarnes2004Ctenophora"/> The name "ctenophora" means "comb-bearing", from the [[Greek Language|Greek]] ''{{lang|grc|κτείς}}'' (stem-form {{lang|grc|κτεν-}}) meaning "comb" and the Greek suffix ''{{lang|grc|-φορος}}'' meaning "carrying".<ref>{{cite encyclopedia | last1=Trumble |first1=W. | last2=Brown |first2=L. | year=2002 | title=Ctenophore | dictionary=Shorter Oxford English Dictionary | publisher=Oxford University Press}}</ref>{{-}}

{| class="wikitable" style="margin-left:4px"
|+ Comparison with other major animal groups
! &nbsp; 
! [[Sponges]]<ref name=RuppertBarnes2004Porifera>{{cite book | last1=Ruppert |first1=E.E. | last2=Fox |first2=R.S. | last3=Barnes |first3=R.D. | year=2004 | title=Invertebrate Zoology | edition=7 | pages=[https://archive.org/details/isbn_9780030259821/page/76 76–97] | publisher=Brooks / Cole | isbn=978-0-03-025982-1 | url=https://archive.org/details/isbn_9780030259821/page/76}}</ref><ref name=Bergquist2001PoriferaInAnderson>{{cite book | last=Bergquist |first=P.R. | year=1998 | chapter=Porifera | pages=10–27 | editor-last=Anderson |editor-first=D.T. | title=Invertebrate Zoology | publisher=Oxford University Press | isbn=978-0-19-551368-4}}</ref>
! [[Cnidarian]]s<ref name=Hinde2001CnidariaAndCtenophoraInAnderson/><ref name=RuppertBarnes2004CnidariaGeneral/><ref name=Moroz2014>{{cite journal |last1=Moroz |first1=Leonid L. |last2=Kocot |first2=Kevin M. |last3=Citarella |first3=Mathew R. |last4=Dosung |first4=Sohn |last5=Norekian |first5=Tigran P. |last6=Povolotskaya |first6=Inna S. |last7=Grigorenko |first7=Anastasia P. |last8=Dailey |first8=Christopher |last9=Berezikov |first9=Eugene |last10=Buckley |first10=Katherine M. |last11=Ptitsyn |first11=Andrey |last12=Reshetov |first12=Denis |last13=Mukherjee |first13=Krishanu |last14=Moroz |first14=Tatiana P. |last15=Bobkova |first15=Yelena |last16=Yu |first16=Fahong |last17=Kapitonov |first17=Vladimir V. |last18=Jurka |first18=Jerzy |last19=Bobkov |first19=Yuri V. |last20=Swore |first20=Joshua J. |last21=Girardo |first21=David O. |last22=Fodor |first22=Alexander |last23=Gusev |first23=Fedor |last24=Sanford |first24=Rachel |last25=Bruders |first25=Rebecca |last26=Kittler |first26=Ellen |last27=Mills |first27=Claudia E. |last28=Rast |first28=Jonathan P. |last29=Derelle |first29=Romain |last30=Solovyev |first30=Victor V. |last31=Kondrashov |first31=Fyodor A. |last32=Swalla |first32=Billie J. |last33=Sweedler |first33=Jonathan V. |last34=Rogaev |first34=Evgeny I. |last35=Halanych |first35=Kenneth M. |last36=Kohn |first36=Andrea B. |display-authors=6 |date=June 2014 |title=The ctenophore genome and the evolutionary origins of neural systems |journal=[[Nature (journal)|Nature]] |volume=510 |issue=7503 |pages=109–114 |doi=10.1038/nature13400 |pmid=24847885 |pmc=4337882 |bibcode=2014Natur.510..109M}}</ref>
! Ctenophores<ref name=Hinde2001CnidariaAndCtenophoraInAnderson/><ref name=RuppertBarnes2004Ctenophora>
{{cite book | last1=Ruppert |first1=E.E. | last2=Fox |first2=R.S. | last3=Barnes |first3=R.D. | year=2004
 | title=Invertebrate Zoology | edition=7 | publisher=Brooks/Cole | isbn=978-0-03-025982-1 | pages=[https://archive.org/details/isbn_9780030259821/page/182 182–195] | url=https://archive.org/details/isbn_9780030259821/page/182}}</ref>
! [[Bilateria]]<ref name="Hinde2001CnidariaAndCtenophoraInAnderson"/>
|- align="center"
! [[Cnidocyte]]s
| No 
| Yes
|colspan="2"| Only in some species<br/>{{small|(obtained from ingested cnidarians)}} 
|- align="center"
! [[microRNA]]
|colspan="2"| Yes 
| No 
| Yes
|- align="center"
! [[Hox gene]]s
| No 
| Yes 
| No 
| Yes
|- align="center"
! [[Colloblast]]s
|colspan="2"| No 
| In most species<ref name=MillsNotesFromExpert>
{{cite web | last=Mills |first=C.E. | title=Ctenophores – some notes from an expert | publisher=[[University of Washington]] | place=Seattle, WA | url=http://faculty.washington.edu/cemills/Ctenophores.html | access-date=2009-02-05}}</ref>
| No
|- align="center"
! [[Digestive system|Digestive]] and [[circulatory system|circulatory]] [[organ (anatomy)|organ]]s
|colspan="3"| No 
| Yes
|- align="center"
! Anal pores
|colspan="2"| No 
| Yes 
| Mostly Yes
|- align="center"
! Number of main cell layers
|colspan="2"| Two, with jelly-like layer between them 
| Debate about whether two<ref name=Hinde2001CnidariaAndCtenophoraInAnderson/> or three<ref name=SeipelSchmid2005EvolutionOfStriatedMuscle/><ref name=RuppertBarnes2004Ctenophora/>
| Three
|- align="center"
! {{small|Cells in each layer bound together}}
| No, except that [[Homoscleromorpha]] have [[basement membrane]]s<ref name=ExpositoCluzelEtAl2002EvolutionOfCollagens>
{{cite journal | last1=Exposito | first1=J-Y. | last2=Cluzel | first2=C. | last3=Garrone | first3=R. | last4=Lethias | first4=C. | year=2002 | title=Evolution of collagens | journal=The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology | volume=268 | issue=3 | pages=302–316 | doi=10.1002/ar.10162 | doi-access=free | pmid=12382326 | s2cid=12376172}}</ref>
|colspan="3"| Yes: Inter-cell connections; basement membranes
|- align="center"
! [[Sensory system|Sensory]] organs
| No
|colspan="3"| Yes
|- align="center"
! Eyes<br/>{{small|(e.g. [[ocelli]])}}
| No
| Yes
| No
| Yes
|- align="center"
! Apical organ
| No
|colspan="2"| Yes
| In species with primary ciliated larvae
|- align="center"
! Cell abundance<br/>{{small|in middle "jelly" layer}}
| Many
|colspan="2"| Few
|rowspan="2"| {{small|{{grey|[not applicable]}} }}
|- align="center"
! Outer layer cells
| can move inwards and change functions
|colspan="2"| do not move or change
|- align="center"
! Nervous system
| No
|colspan="2"| Yes, simple
| Simple to complex
|- align="center"
! [[Muscle]]s
| None
| Mostly epitheliomuscular
| Mostly [[myoepithelial]]
| Mostly [[myocyte]]s
|}

== Description ==

[[File:Comb Jelly, Shedd Aquarium, Chicago.webmhd.webm|thumb|Comb jelly, [[Shedd Aquarium]], Chicago]]

For a phylum with relatively few species, ctenophores have a wide range of body plans.<ref name="RuppertBarnes2004Ctenophora"/> Coastal species need to be tough enough to withstand waves and swirling sediment particles, while some oceanic species are so fragile that it is very difficult to capture them intact for study.<ref name="MillsNotesFromExpert" /> In addition, oceanic species do not preserve well,<ref name="MillsNotesFromExpert" /> and are known mainly from photographs and from observers' notes.<ref name="Horita2000LobatolampeaTetragona">{{cite journal | last=Horita |first=T. | date=March 2000
| title=An undescribed lobate ctenophore, ''Lobatolampea tetragona'' gen. nov. & spec. nov., representing a new family, from Japan | journal=Zoologische Mededelingen | volume=73 | issue=30| pages=457–464 | url=http://www.repository.naturalis.nl/document/44309 | access-date=2009-01-03 }}</ref> Hence most attention has until recently concentrated on three coastal [[genus|genera]] – ''[[Pleurobrachia]]'', ''[[Beroe (ctenophore)|Beroe]]'' and ''[[Mnemiopsis]]''.<ref name="MillsNotesFromExpert" /><ref name="Haddock204goldenAgeofGelata">{{cite journal |last1=Haddock |first1=Steven H. D. |author-link=Steven Haddock |title=A golden age of gelata: past and future research on planktonic ctenophores and cnidarians |journal=Hydrobiologia |date=November 2004 |volume=530-531 |issue=1–3 |pages=549–556 |doi=10.1007/s10750-004-2653-9 |bibcode=2004HyBio.530..549H |s2cid=17105070 }}</ref> At least two textbooks base their descriptions of ctenophores on the [[Cydippida|cydippid]] ''Pleurobrachia''.<ref name="Hinde2001CnidariaAndCtenophoraInAnderson" /><ref name="RuppertBarnes2004Ctenophora" />

Since the body of many species is ''almost'' [[radial symmetry|radially symmetrical]], the main axis is [[Mouth|oral]] to [[aboral]] (from the mouth to the opposite end). However, since only two of the canals near the [[statocyst]] terminate in [[anus|anal]] pores, ctenophores have no mirror-symmetry, although many have rotational symmetry. In other words, if the animal rotates in a half-circle it looks the same as when it started.<ref>{{cite journal |last1=Martindale |first1=M. Q. |last2=Henry |first2=J. Q. |title=Intracellular Fate Mapping in a Basal Metazoan, the Ctenophore ''Mnemiopsis leidyi'', Reveals the Origins of Mesoderm and the Existence of Indeterminate Cell Lineages |journal=Developmental Biology |volume=214 |issue=2|date=October 1999|pmid=10525332 |pages=243–257 |doi=10.1006/dbio.1999.9427 |doi-access=free}}</ref>

===Body layers ===

[[File:Ctenophore diagram - en.svg|thumb|alt=Anatomy of Cydippid Ctenophore|Anatomy of Cydippid Ctenophore]]

Like those of [[cnidaria]]ns, ([[jellyfish]], [[sea anemone]]s, etc.), ctenophores' bodies consist of a relatively thick, jelly-like [[mesoglea]] sandwiched between two [[epithelium|epithelia]], layers of [[cell (biology)|cells]] bound by inter-cell connections and by a fibrous [[basement membrane]] that they [[secrete]].<ref name="Hinde2001CnidariaAndCtenophoraInAnderson" /><ref name="RuppertBarnes2004Ctenophora"/> The epithelia of ctenophores have two layers of cells rather than one, and some of the cells in the upper layer have several [[cilia]] per cell.<ref name="RuppertBarnes2004Ctenophora"/>

The outer layer of the [[Epidermis (zoology)|epidermis]] (outer skin) consists of: sensory cells; cells that secrete [[mucus]], which protects the body; and interstitial cells, which can transform into other types of cell. In specialized parts of the body, the outer layer contains [[colloblast]]s along the surface of tentacles, used in capturing prey, or cells bearing multiple large cilia for locomotion. The inner layer of the epidermis contains a [[nerve net]], and myoepithelial cells that act as [[muscle]]s.<ref name="RuppertBarnes2004Ctenophora"/>

The internal cavity forms: a mouth that can usually be closed by muscles; a [[pharynx]] ("throat"); a wider area in the center that acts as a [[stomach]]; and a system of internal canals. These branch through the mesoglea to the most active parts of the animal. The inner surface of the cavity is lined with an [[epithelium]], the [[gastrodermis]]. The mouth and pharynx have both [[cilia]] and muscles. In other parts of the canal system, the gastrodermis is different on the sides nearest to and furthest from the organ that it supplies. The nearer side is composed of tall nutritive cells that store nutrients in [[vacuole]]s (internal compartments), [[germ cell]]s that produce eggs or sperm, and [[photocytes]] that produce [[bioluminescence]]. The side furthest from the organ is covered with ciliated cells that circulate water through the canals, punctuated by ciliary rosettes, pores surrounded by double whorls of cilia and connected to the mesoglea.<ref name="RuppertBarnes2004Ctenophora"/>

=== Feeding, excretion and respiration ===

When prey is swallowed, it is liquefied in the [[pharynx]] by [[enzyme]]s and muscular contractions of the pharynx. The resulting slurry is wafted through the canal system by the beating of the [[cilia]], and digested by the nutritive cells. The ciliary rosettes may help to transport nutrients to muscles in the mesoglea. The [[anus|anal]] pores may eject unwanted small particles, but most unwanted matter is regurgitated via the mouth.<ref name="RuppertBarnes2004Ctenophora"/>

Little is known about how ctenophores get rid of waste products produced by the cells. The ciliary rosettes in the [[gastrodermis]] may help to remove wastes from the mesoglea, and may also help to adjust the animal's [[buoyancy]] by pumping water into or out of the mesoglea.<ref name="RuppertBarnes2004Ctenophora"/>

=== Locomotion ===

The outer surface bears usually eight comb rows, called swimming-plates, which are used for swimming. The rows are oriented to run from near the mouth (the "oral pole") to the opposite end (the "aboral pole"), and are spaced more or less evenly around the body,<ref name="Hinde2001CnidariaAndCtenophoraInAnderson" /> although spacing patterns vary by species and in most species the comb rows extend only part of the distance from the aboral pole towards the mouth. The "combs" (also called "ctenes" or "comb plates") run across each row, and each consists of thousands of unusually long cilia, up to {{convert|2|mm|in|2|sp=us}}. Unlike conventional cilia and flagella, which has a [[Axoneme|filament]] structure arranged in a 9 + 2 pattern, these cilia are arranged in a 9 + 3 pattern, where the extra compact filament is suspected to have a supporting function.<ref>{{cite journal |pmc=2224992 |pmid=13681575 |volume=9 |issue=2 |title=The fine structure of the cilia from ctenophore swimming-plates |journal=The Journal of Biophysical and Biochemical Cytology |pages=383–94 |last1=Afzelius |first1=BA |doi=10.1083/jcb.9.2.383 |year=1961}}</ref> These normally beat so that the propulsion stroke is away from the mouth, although they can also reverse direction. Hence ctenophores usually swim in the direction in which the mouth is eating, unlike [[jellyfish]].<ref name="RuppertBarnes2004Ctenophora" /> 

It is uncertain how ctenophores control their buoyancy, but some species rely on [[osmotic pressure]] to adapt to water of different densities.<ref>{{cite journal |last1=Mills |first1=Claudia E. |title=Density is Altered in Hydromedusae and Ctenophores in Response to Changes in Salinity |journal=The Biological Bulletin |date=February 1984 |volume=166 |issue=1 |pages=206–215 |doi=10.2307/1541442 |jstor=1541442 |url=https://www.biodiversitylibrary.org/part/37655 }}</ref> Their body fluids are normally as [[concentration|concentrated]] as seawater. If they enter less dense brackish water, the ciliary rosettes may pump this into the [[mesoglea]] to maintain buoyancy. Conversely, if they move from brackish to full-strength seawater, the rosettes may pump water out of the mesoglea.<ref name="RuppertBarnes2004Ctenophora"/>

=== Nervous system and senses ===

Ctenophores have no [[brain]] or [[central nervous system]], but have a subepidermal [[nerve net]] that forms a ring round the mouth and is densest near structures such as the comb rows, pharynx, tentacles and the sensory complex furthest from the mouth.<ref name="RuppertBarnes2004Ctenophora"/> Nerve cells communicate by two different methods; some of the neurons have [[synapses|synaptic connections]], but those in the nerve net are highly distinctive by being fused into a [[syncytium]].<ref name=Burkhardt2023>{{cite journal |last1=Burkhardt |first1=Pawel |last2=Colgren |first2=Jeffrey |last3=Medhus |first3=Astrid |last4=Digel |first4=Leonid |last5=Naumann |first5=Benjamin |last6=Soto-Angel |first6=Joan |last7=Nordmann |first7=Eva-Lena |last8=Sachkova |first8=Maria |last9=Kittelmann |first9=Maike |display-authors=6 |title=Syncytial nerve net in a ctenophore adds insights on the evolution of nervous systems |journal=[[Science (journal)|Science]] |date=20 April 2023 |volume=380 |issue=6642 |pages=293–297 |doi=10.1126/science.ade5645 |pmid=37079688 |bibcode=2023Sci...380..293B |s2cid=258239574 |hdl=11250/3149299 |hdl-access=free |pmc=7617566 }}</ref> Fossils show that Cambrian species had a more complex nervous system, with long nerves which connected with a ring around the mouth. The only ctenophores with long nerves today is ''[[Euplokamididae|Euplokamis]]'' in the order Cydippida.<ref>{{cite journal |last1=Parry |first1=Luke A. |last2=Lerosey-Aubril |first2=Rudy |last3=Weaver |first3=James C. |last4=Ortega-Hernández |first4=Javier |title=Cambrian comb jellies from Utah illuminate the early evolution of nervous and sensory systems in ctenophores |journal=iScience |date=2021 |volume=24 |issue=9 |article-number=102943 |doi=10.1016/j.isci.2021.102943 |pmid=34522849 |pmc=8426560 |bibcode=2021iSci...24j2943P |doi-access=free}}</ref> Their nerve cells arise from the same [[progenitor cell]]s as the colloblasts.<ref>{{cite journal |last1=Pennisi |first1=Elizabeth |author-link=Elizabeth Pennisi |title=The gluey tentacles of comb jellies may have revealed when nerve cells first evolved |journal=[[Science (journal)|Science]] |date=10 January 2019 |doi=10.1126/science.aaw6288 |s2cid=92852830 }}</ref>

In addition, there is a less organized mesogleal nerve net consisting of single neurites. The largest single sensory feature is the [[aboral]] organ (at the opposite end from the mouth), which is underlined with its own nerve net.<ref>[http://ryanlab.whitney.ufl.edu/pdfs/doi_10.1016_j.zool.2014.06.001.pdf Did the ctenophore nervous system evolve independently?]</ref> This organ's main component is a [[statocyst]], a balance sensor consisting of a statolith, a tiny grain of calcium carbonate, supported on four bundles of [[cilia]], called "balancers", that sense its orientation. The statocyst is protected by a transparent dome of long, immobile cilia. A ctenophore does not automatically try to keep the statolith resting equally on all the balancers. Instead, its response is determined by the animal's "mood", in other words, the overall state of the nervous system. For example, if a ctenophore with trailing tentacles captures prey, it often puts some comb rows into reverse, spinning the mouth towards the prey.<ref name="RuppertBarnes2004Ctenophora" />

The ciliated larvae in cnidarians and bilaterians appear to share an ancient and common origin.<ref>{{cite journal |last1=Marlow |first1=Heather |last2=Tosches |first2=Maria Antonietta |last3=Tomer |first3=Raju |last4=Steinmetz |first4=Patrick R. |last5=Lauri |first5=Antonella |last6=Larsson |first6=Tomas |last7=Arendt |first7=Detlev |date=29 January 2014 |title=Larval body patterning and apical organs are conserved in animal evolution |journal=[[BMC Biology]] |volume=12 |issue=1 |page=7 |doi=10.1186/1741-7007-12-7 |pmid=24476105 |pmc=3939940 |doi-access=free }}</ref> The larvae's apical organ is involved in the formation of the nervous system.<ref>{{cite journal |last1=Nielsen |first1=Claus |title=Larval nervous systems: true larval and precocious adult |journal=[[Journal of Experimental Biology]] |date=15 February 2015 |volume=218 |issue=4 |pages=629–636 |doi=10.1242/jeb.109603 |pmid=25696826 |bibcode=2015JExpB.218..629N |s2cid=3151957 |doi-access=free }}</ref> The aboral organ of comb jellies is not homologous with the apical organ in other animals, and the formation of their nervous system has therefore a different embryonic origin.<ref>{{cite journal |last1=Nielsen |first1=Claus |title=Early animal evolution: a morphologist's view |journal=Royal Society Open Science |date=July 2019 |volume=6 |issue=7 |article-number=190638 |doi=10.1098/rsos.190638 |pmid=31417759 |pmc=6689584 |bibcode=2019RSOS....690638N }}</ref>

Ctenophore nerve cells and nervous system have distinctive biochemistry. They lack the genes and enzymes required to manufacture neurotransmitters like [[serotonin]], [[dopamine]], [[Biological functions of nitric oxide|nitric oxide]], [[octopamine]], [[noradrenaline]], and others, seen in all other animals with a nervous system, with the genes coding for the receptors for each of these neurotransmitters missing.<ref>{{cite web |first=Douglas |last=Fox |title=Aliens in our midst |url=https://aeon.co/essays/what-the-ctenophore-says-about-the-evolution-of-intelligence |date=1 August 2017 |work=[[Aeon (digital magazine)|Aeon]] |access-date=1 August 2017}}</ref> Monofunctional [[catalase]] (CAT), one of the three major families of antioxidant enzymes that target [[hydrogen peroxide]], an important signaling molecule for synaptic and neuronal activity, is also absent, most likely due to gene loss.<ref>{{Cite journal |last1=Hewitt |first1=Olivia H. |last2=Degnan |first2=Sandie M. |date=2023-02-13 |title=Antioxidant enzymes that target hydrogen peroxide are conserved across the animal kingdom, from sponges to mammals |journal=Scientific Reports |volume=13 |issue=1 |page=2510 |doi=10.1038/s41598-023-29304-6 |pmid=36781921 |pmc=9925728 |bibcode=2023NatSR..13.2510H |s2cid=256811787}}</ref> They use [[L-glutamate]] as a [[neurotransmitter]], and have a distinctively high number of ionotropic glutamate receptors and genes for glutamate synthesis and transport.<ref name="Norekian">{{cite journal |last1=Norekian |first1=Tigran P. |last2=Moroz |first2=Leonid L. |title=Neural system and receptor diversity in the ctenophore Beroe abyssicola |journal=[[Journal of Comparative Neurology]] |date=15 August 2019 |volume=527 |issue=12 |pages=1986–2008 |doi=10.1002/cne.24633 |pmid=30632608 |doi-access=free }}</ref> The genomic content of the nervous system is the smallest of any animal, and could represent the minimum genetic requirements for a functional nervous system.<ref>{{cite book |doi=10.1093/acprof:oso/9780199682201.003.0006 |chapter=Ctenophora |title=Structure and Evolution of Invertebrate Nervous Systems |year=2015 |last1=Simmons |first1=David K. |last2=Martindale |first2=Mark Q. |pages=48–55 |publisher=[[Oxford University Press]] |isbn=978-0-19-968220-1 }}</ref> The presence of directly fused neurons without synapses suggests that ctenophores might form a sister group to other metazoans, having developed a nervous system independently.<ref name="Burkhardt2023"/> If so, nervous systems may have either been lost in sponges and placozoans, or arisen more than once among metazoans.<ref name="Jékely2015">{{cite journal |last1=Jákely |first1=Gáspár |last2=Paps |first2=Jordi |last3=Nielsen |first3=Claus |year=2015 |title=The phylogenetic position of ctenophores and the origin(s) of nervous systems |journal=[[EvoDevo]] |volume=6 |issue=1 |page=1 |doi=10.1186/2041-9139-6-1 |pmid=25905000 |pmc=4406211 |doi-access=free }}</ref>

===Reproduction and development===

[[File:Juvenile Bolinopsis ctenophore.jpg|thumb|Cydippid larva of ''Bolinopsis'' sp., a few millimetres long]]

Adults of most species can regenerate tissues that are damaged or removed,<ref>{{cite journal
|last=Martindale |first=M. Q. |title=The ontogeny and maintenance of adult symmetry properties in the ctenophore, ''Mnemiopsis mccradyi''|journal=Developmental Biology |date=December 1986|volume=118 |issue=2 |pages=556–576 |pmid=2878844|doi=10.1016/0012-1606(86)90026-6}}</ref> although only platyctenids reproduce by [[cloning]], splitting off from the edges of their flat bodies fragments that develop into new individuals.<ref name="RuppertBarnes2004Ctenophora" /> Lab research on ''Mnemiopsis leidyi'' also show that when two individuals have parts of their bodies removed, they are able to fuse together, including their nervous and digestive systems, even when the two individuals are genetically different; a phenomenon so far only found in comb jellies.<ref>[https://www.cell.com/current-biology/abstract/S0960-9822(24)01023-6 Rapid physiological integration of fused ctenophores]</ref>

The [[Most recent common ancestor|last common ancestor (LCA)]] of the ctenophores was [[hermaphrodite|hermaphroditic]].<ref name="Sasson">{{cite journal |last1=Sasson |first1=Daniel A. |last2=Ryan |first2=Joseph F. |title=A reconstruction of sexual modes throughout animal evolution |journal=BMC Evolutionary Biology |date=December 2017 |volume=17 |issue=1 |page=242 |doi=10.1186/s12862-017-1071-3 |pmid=29207942 |pmc=5717846 |doi-access=free |bibcode=2017BMCEE..17..242S }}</ref> Some are simultaneous hermaphrodites, which can produce both eggs and sperm at the same time, while others are sequential hermaphrodites, in which the eggs and sperm mature at different times. There is no [[metamorphosis]].<ref>{{cite journal |doi=10.1073/pnas.2122052119 |title=Ctenophores are direct developers that reproduce continuously beginning very early after hatching |year=2022 |last1=Edgar |first1=Allison |last2=Ponciano |first2=José Miguel |last3=Martindale |first3=Mark Q. |journal=Proceedings of the National Academy of Sciences of the USA |volume=119 |issue=18 |article-number=e2122052119 |doi-access=free |pmid=35476523 |pmc=9170174 |bibcode=2022PNAS..11922052E }}</ref> At least three species are known to have evolved separate sexes ([[dioecy]]); ''Ocyropsis crystallina'' and ''Ocyropsis maculata'' in the genus [[Ocyropsis]] and ''Bathocyroe fosteri'' in the genus [[Bathocyroe]].<ref name="Harbison">{{cite journal |last1=Harbison |first1=G. R. |last2=Miller |first2=R. L. |title=Not all ctenophores are hermaphrodites. Studies on the systematics, distribution, sexuality and development of two species of Ocyropsis |journal=[[Marine Biology (journal)|Marine Biology]] |date=February 1986 |volume=90 |issue=3 |pages=413–424 |doi=10.1007/bf00428565 |bibcode=1986MarBi..90..413H |s2cid=83954780 }}</ref> The [[gonad]]s are located in the parts of the internal canal network under the comb rows, and eggs and sperm are released via pores in the epidermis. Fertilization is generally [[external fertilization|external]], but platyctenids use internal fertilization and keep the eggs in brood chambers until they hatch. Self-fertilization has occasionally been seen in species of the genus ''[[Mnemiopsis]]'',<ref name="RuppertBarnes2004Ctenophora"/> and most hermaphroditic species are presumed to be self-fertile.<ref name="MillsNotesFromExpert"/>

Development of the fertilized eggs is direct; there is no distinctive larval form. Juveniles of all groups are generally [[plankton]]ic, and most species resemble miniature adult cydippids, gradually developing their adult body forms as they grow. In the genus ''Beroe'', however, the juveniles have large mouths and, like the adults, lack both tentacles and tentacle sheaths. In some groups, such as the flat, bottom-dwelling platyctenids, the juveniles behave more like true larvae. They live among the plankton and thus occupy a different [[ecological niche]] from their parents, only attaining the adult form by a more radical [[ontogeny]]<ref name="RuppertBarnes2004Ctenophora"/> after dropping to the sea-floor.<ref name="MillsNotesFromExpert"/>

At least in some species, juvenile ctenophores appear capable of producing small quantities of eggs and sperm while they are well below adult size, and adults produce eggs and sperm for as long as they have sufficient food. If they run short of food, they first stop producing eggs and sperm, and then shrink in size. When the food supply improves, they grow back to normal size and then resume reproduction. These features enable ctenophores to increase their populations very quickly.<ref name="MillsNotesFromExpert" /> Members of the Lobata and Cydippida have a reproduction form called dissogeny; two sexually mature stages, first as larva and later as juveniles and adults. During their time as larvae they release gametes periodically. After their first reproductive period is over they do not produce more gametes until later. A population of ''Mertensia ovum'' in the central [[Baltic Sea]] have become [[Neoteny|paedogenetic]], and consist solely of sexually mature larvae less than 1.6&nbsp;mm.<ref>{{cite journal|pmc=4971632 |pmid=27489613 |doi=10.1186/s13227-016-0051-9 |volume=7 |title=Developmental expression of 'germline'- and 'sex determination'-related genes in the ctenophore ''Mnemiopsis leidyi'' |year=2016 |journal=[[EvoDevo]] |article-number=17 |last1=Reitzel |first1=A. M. |last2=Pang |first2=K. |last3=Martindale |first3=M. Q. |doi-access=free }}</ref><ref>{{cite journal|pmc=3440961 |pmid=22535640 |doi=10.1098/rsbl.2012.0163 |volume=8 |issue=5 |title=Ctenophore population recruits entirely through larval reproduction in the central Baltic Sea |year=2012 |journal=Biology Letters |pages=809–12 |last1=Jaspers |first1=C. |last2=Haraldsson |first2=M. |last3=Bolte |first3=S. |last4=Reusch |first4=T. B. |last5=Thygesen |first5=U. H. |last6=Kiørboe |first6=T.}}</ref>

In ''Mnemiopsis leidyi'', nitric oxide (NO) signaling is present both in adult tissues and differentially expressed in later embryonic stages suggesting the involvement of NO in developmental mechanisms.<ref>{{cite journal|doi=10.3389/fnins.2023.1125433 |volume=17 |title=Nitric oxide signaling in ctenophores |year=2023 |journal=Front. Neurosci. |article-number=1125433 |last1=Moroz |first1=Leonid |last2=Mukherjee |first2=Krishanu |last3=Romanova |first3=Daria |pmid=37034176 |pmc=10073611 |doi-access=free }}</ref> The mature form of the same species is also able to revert back to the cydippid stage when triggered by environmental stressors.<ref>[https://www.zmescience.com/science/news-science/this-benjamin-button-like-jellyfish-can-age-in-reverse-from-adult-to-juvenile/ This Benjamin Button-like Jellyfish Can Age in Reverse, From Adult to Juvenile]</ref>

===Colors and bioluminescence===

[[File:LightRefractsOf comb-rows of ctenophore Mertensia ovum.jpg|thumb|Light [[diffraction|diffracting]] along the comb rows of a ''[[Mertensia ovum]]'', left tentacle deployed, right tentacle retracted]]

Most ctenophores that live near the surface are mostly colorless and almost transparent. However some deeper-living species are strongly pigmented, for example the species known as "Tortugas red"<ref name="MillsCtenoList">{{cite web |url=http://faculty.washington.edu/cemills/Ctenolist.html |access-date=2009-02-10 |title=Phylum Ctenophora: list of all valid scientific names |last=Mills |first=C. E. |date=May 2007}}</ref> (see illustration here), which has not yet been formally described.<ref name="MillsNotesFromExpert" /> Platyctenids generally live attached to other sea-bottom organisms, and often have similar colors to these host organisms.<ref name="MillsNotesFromExpert"/> The gut of the deep-sea genus ''[[Bathocyroe]]'' is red, which hides the [[bioluminescence]] of [[copepod]]s it has swallowed.<ref name="Haddock2007ComparativeFeeding"/>

The comb rows of most planktonic ctenophores produce a rainbow effect, which is not caused by [[bioluminescence]] but by the [[diffraction|scattering of light]] as the combs move.<ref name="MillsNotesFromExpert" /><ref>{{cite journal |last1=Welch |first1=Victoria |last2=Vigneron |first2=Jean Pol |last3=Lousse |first3=Virginie |last4=Parker |first4=Andrew |title=Optical properties of the iridescent organ of the comb-jellyfish Beroë cucumis (Ctenophora) |journal=Physical Review E |date=14 April 2006 |volume=73 |issue=4 |article-number=041916 |doi=10.1103/PhysRevE.73.041916 |pmid=16711845 |bibcode=2006PhRvE..73d1916W }}</ref><!-- e pagination --> Most species are also bioluminescent, but the light is usually blue or green and can only be seen in darkness.<ref name="MillsNotesFromExpert" /> However some significant groups, including all known platyctenids and the cydippid [[genus]] ''[[Pleurobrachia]]'', are incapable of bioluminescence.<ref>{{cite journal |last1=Haddock |first1=Steven H.D. |last2=Case |first2=J.F. |date=December 1995 |title=Not all ctenophores are bioluminescent: Pleurobrachia |journal=[[Biological Bulletin]] |volume=189 |issue=3 |pages=356–362 |doi=10.2307/1542153 |pmid=29244577 |jstor=1542153 |url=https://www.biodiversitylibrary.org/part/19935 }}</ref>

When some species, including ''[[Bathyctena chuni]]'', ''[[Euplokamis stationis]]'' and ''[[Eurhamphaea vexilligera]]'', are disturbed, they produce secretions (ink) that luminesce at much the same [[wavelength]]s as their bodies. Juveniles will luminesce more brightly in relation to their body size than adults, whose luminescence is diffused over their bodies. Detailed statistical investigation has not suggested the function of ctenophores' bioluminescence nor produced any [[correlation]] between its exact color and any aspect of the animals' environments, such as depth or whether they live in coastal or mid-ocean waters.<ref>{{cite journal |last1=Haddock |first1=Steven H.D. |last2=Case |first2=J.F. |date=8 April 1999 |title=Bioluminescence spectra of shallow and deep-sea gelatinous zooplankton: Ctenophores, medusae and siphonophores |journal=[[Marine Biology (journal)|Marine Biology]] |volume=133 |issue=3 |pages=571–582 |doi=10.1007/s002270050497 |bibcode=1999MarBi.133..571H |s2cid=14523078 }}</ref>

In ctenophores, bioluminescence is caused by the activation of calcium-activated proteins named [[photoproteins]] in cells called [[photocytes]], which are often confined to the meridional canals that underlie the eight comb rows. In the genome of ''[[Mnemiopsis leidyi]]'' ten genes encode photoproteins. These genes are co-expressed with [[opsin]] genes in the developing photocytes of ''Mnemiopsis leidyi'', raising the possibility that light production and light detection may be working together in these animals.<ref name=SchnitzlerPang2012>{{cite journal |last1=Schnitzler |first1=Christine E. |last2=Pang |first2=Kevin |last3=Powers |first3=Meghan L. |last4=Reitzel |first4=Adam M. |last5=Ryan |first5=Joseph F. |last6=Simmons |first6=David |last7=Tada |first7=Takashi |last8=Park |first8=Morgan |last9=Gupta |first9=Jyoti |last10=Brooks |first10=Shelise Y. |last11=Blakesley |first11=Robert W. |last12=Yokoyama |first12=Shozo |last13=Haddock |first13=Steven H.D. |last14=Martindale |first14=Mark Q. |last15=Baxevanis |first15=Andreas D. |display-authors=6 |year=2012 |title=Genomic organization, evolution, and expression of photoprotein and opsin genes in Mnemiopsis leidyi: a new view of ctenophore photocytes |journal=[[BMC Biology]] |volume=10 |issue=1 |page=107 |doi=10.1186/1741-7007-10-107 |doi-access=free |pmid=23259493 |pmc=3570280 }}</ref>

== Ecology ==

[[File:Ctenophore.jpg|thumb|"Tortugas red", with trailing tentacles and clearly visible sidebranches, or tentilla]]

=== Distribution ===

Ctenophores are found in most marine environments: from polar waters at −2&nbsp;°C to the tropics at 30&nbsp;°C; near coasts and in mid-ocean; from the surface waters to the ocean depths at more than 7000 meters.<ref>{{Cite journal |last1=Winnikoff |first1=Jacob R. |last2=Haddock |first2=Steven H. D. |last3=Budin |first3=Itay |date=2021-11-01 |title=Depth- and temperature-specific fatty acid adaptations in ctenophores from extreme habitats |journal=[[Journal of Experimental Biology]] |volume=224 |issue=21 |pages=jeb242800 |doi=10.1242/jeb.242800 |pmc=8627573 |pmid=34676421 |bibcode=2021JExpB.224B2800W }}</ref> The best-understood are the [[genus|genera]] ''Pleurobrachia'', ''Beroe'' and ''[[Mnemiopsis]]'', as these [[plankton]]ic coastal forms are among the most likely to be collected near shore.<ref name="Haddock204goldenAgeofGelata" /><ref name="Haddock2007ComparativeFeeding" />

In 2013 ''Mnemiopsis'' was recorded in lake Birket Qarun, and in 2014 in [[lake El Rayan]] II, both near [[Faiyum]] in Egypt, where they were accidentally introduced by the transport of fish (mullet) fry. Though many species prefer brackish waters like estuaries and coastal lagoons in open connection with the sea, this was the first record from an inland environment. Both lakes are saline, with Birket Qarun being hypersaline, and shows that some ctenophores can establish themselves in saline limnic environments without connection to the ocean. In the long run, it is not expected the populations will survive. The two limiting factors in saline lakes are availability of food and a varied diet, and high temperatures during hot summers. Because a parasitic isopod, ''Livoneca redmanii'', was introduced at the same time, it is difficult to say how much of the ecological impact of invasive species is caused by the ctenophore alone.<ref name="El-Shabrawy Dumont 2016 pp. 21–24">{{cite journal |first1=Gamal |last1=el-Shabrawy |first2=Henri |last2=Dumont |year=2016 |title=First record of a ctenophore in lakes: The comb-jelly ''Mnemiopsis leidyi'' ({{small|A. Agassiz, 1865}}) invades the Fayum, Egypt |journal=BioInvasions Records |volume=5 |issue=1 |pages=21–24 |s2cid=90377075 |doi=10.3391/bir.2016.5.1.04 |doi-access=free}}</ref><ref>{{cite journal |last1=Mohammed-Geba |first1=Khaled |last2= Sheir |first2=Sherin |last3=Aguilar |first3=Robert |last4= Ogburn |first4=Matthew |last5= Hines |first5=Anson |last6= Khalafallah |first6=Hussain |last7=el-Kattan |first7=Ahmed |last8= Hassab el-Nabi |first8=Sobhy |last9=Galal-Khallaf |first9=Asmaa |display-authors=6 |year=2019 |title=Molecular and morphological confirmation of an invasive American isopod; ''Livoneca redmanii'' ({{small|Leach, 1818}}), from the Mediterranean region to Lake Qaroun, Egypt |journal=Egyptian Journal of Aquatic Biology and Fisheries |volume=23 |issue=4 |pages=251–273 |doi=10.21608/ejabf.2019.54062 |url=https://journals.ekb.eg/article_54062_2d04ff7853ceefc5fbb3f3c23fb58f3c.pdf}}</ref>

=== Prey and predators ===

Almost all ctenophores are [[predator]]s – there are no vegetarians and only one genus that is partly [[parasitic]].<ref name=Haddock2007ComparativeFeeding/> If food is plentiful, they can eat ten times their own weight per day.<ref>{{cite journal |last1=Reeve |first1=M.R. |last2=Walter |first2=M.A. |last3=Ikeda |first3=T. |date=July 1978 |title=Laboratory studies of ingestion and food utilization in lobate and tentaculate ctenophores 1:&nbsp;Ctenophore food utilization |journal=[[Limnology and Oceanography]] |volume=23 |issue=4 |pages=740–751 |doi=10.4319/lo.1978.23.4.0740 |doi-access=free}}</ref> While ''Beroe'' preys mainly on other ctenophores, other surface-water species prey on [[zooplankton]] (planktonic animals) ranging in size from the microscopic, including mollusc and fish larvae, to small adult crustaceans such as [[copepod]]s, [[amphipod]]s, and even [[krill]]. Members of the genus ''[[Haeckelia]]'' prey on [[jellyfish]] and incorporate their prey's [[nematocyst]]s (stinging cells) into their own tentacles instead of [[colloblast]]s.<ref name=MillsNotesFromExpert/> Ctenophores have been compared to [[spider]]s in their wide range of techniques for capturing prey – some hang motionless in the water using their tentacles as "webs", some are ambush predators like Salticid [[jumping spider]]s, and some dangle a sticky droplet at the end of a fine thread, as [[bolas spider]]s do. This variety explains the wide range of body forms in a [[phylum]] with rather few species.<ref name=Haddock2007ComparativeFeeding>{{cite journal |last=Haddock |first=S.H.D. |author-link=Steven Haddock |date=December 2007 |title=Comparative feeding behavior of planktonic ctenophores |journal=[[Integrative and Comparative Biology]] |volume=47 |issue=6 |pages=847–853 |pmid=21669763 |doi=10.1093/icb/icm088 |doi-access=}}</ref> 

The two-tentacled "cydippid" ''Lampea'' feeds exclusively on [[salp]]s, close relatives of sea-squirts that form large chain-like floating colonies, and juveniles of ''Lampea'' attach themselves like parasites to salps that are too large for them to swallow.<ref name=Haddock2007ComparativeFeeding/> Members of the cydippid genus ''[[Pleurobrachia]]'' and the lobate ''[[Bolinopsis]]'' often reach high population densities at the same place and time because they specialize in different types of prey: ''[[Pleurobrachia]]''{{'}}s long tentacles mainly capture relatively strong swimmers such as adult copepods, while ''[[Bolinopsis]]'' generally feeds on smaller, weaker swimmers such as [[rotifer]]s and mollusc and [[crustacean larvae]].<ref>{{cite journal |last1=Costello |first1=J.H. |last2=Coverdale |first2=R. |date=October 1998 |title=Planktonic feeding and evolutionary significance of the lobate body plan within the Ctenophora |journal=[[The Biological Bulletin]] |volume=195 |issue=2 |pages=247–248 |doi=10.2307/1542863 |pmid=28570175 |jstor=1542863}}</ref>

It is often difficult to identify the remains of ctenophores in the guts of possible predators as they are broken down quickly, although the combs sometimes remain intact long enough to provide a clue. [[Chum salmon]], ''Oncorhynchus keta'', digest ctenophores 20 times as fast as an equal weight of [[shrimp]]s; ctenophores can provide the fish with a good diet if there are enough of them around. Some [[jellyfish]] and [[turtle]]s eat large quantities of ctenophores, and jellyfish may temporarily wipe out ctenophore populations. Since ctenophores and jellyfish often have large seasonal variations in population, most fish that prey on them are generalists and may have a greater effect on populations than specialist jelly-eaters. Herbivorous fishes deliberately feed on gelatinous zooplankton during blooms in the Red Sea.<ref>{{cite journal |last1=Bos |first1=A.R. |last2=Cruz-Rivera |first2=E. |last3=Sanad |first3=A.M. |year=2016 |title=Herbivorous fishes ''Siganus rivulatus'' (Siganidae) and ''Zebrasoma desjardinii'' (Acanthuridae) feed on Ctenophora and Scyphozoa in the Red Sea  |journal=[[Marine Biodiversity]] |volume=47 |pages=243–246 |doi=10.1007/s12526-016-0454-9 |s2cid=24694789}}</ref> The larvae of some [[sea anemone]]s are parasites on ctenophores, as are the larvae of some [[flatworm]]s that parasitize fish when they reach adulthood.<ref>{{cite journal |last=Arai |first=Mary Needler |date=June 2005 |title=Predation on pelagic coelenterates: A review |journal=[[Journal of the Marine Biological Association of the United Kingdom]] |volume=85 |issue=3 |pages=523–536 |doi=10.1017/S0025315405011458 |bibcode=2005JMBUK..85..523A |s2cid=86663092}}</ref>

=== Ecological impacts ===

[[File:Medusa010.jpg|thumb|right|''Beroe ovata'' at the surface on the Black Sea coast]]

Ctenophores may balance marine ecosystems by preventing an over-abundance of copepods from eating all the [[phytoplankton]] (planktonic plants),<ref name=ChandyGreene1995PredatoryImpact>{{cite journal |last1=Chandy |first1=Shonali T. |last2=Greene |first2=Charles H. |date=July 1995 |title=Estimating the predatory impact of gelatinous zooplankton |journal=[[Limnology and Oceanography]] |volume=40 |issue=5 |pages=947–955 |doi=10.4319/lo.1995.40.5.0947 |doi-access=free |bibcode=1995LimOc..40..947C}}</ref> which are the dominant marine producers of organic matter from non-organic ingredients.<ref>{{cite journal  |last1=Field |first1=Christopher B. |last2=Behrenfeld |first2=Michael J. |last3=Randerson |first3=James T. |last4=Falkowski |first4=Paul |date=10 July 1998 |title=Primary production of the biosphere: Integrating terrestrial and oceanic components |journal=[[Science (journal)|Science]] |volume=281 |issue=5374 |pages=237–240 |doi=10.1126/science.281.5374.237 |pmid=9657713 |bibcode=1998Sci...281..237F |url=http://www.escholarship.org/uc/item/9gm7074q}}</ref>

On the other hand, in the late 1980s the Western Atlantic ctenophore ''[[Mnemiopsis leidyi]]'' was accidentally [[introduced species|introduced]] into the [[Black Sea]] and [[Sea of Azov]] via the [[ballast tanks]] of ships, and has been blamed for causing sharp drops in fish catches by eating both fish larvae and small crustaceans that would otherwise feed the adult fish.<ref name="ChandyGreene1995PredatoryImpact"/> ''Mnemiopsis'' is well equipped to invade new territories (although this was not predicted until after it so successfully colonized the Black Sea), as it can breed very rapidly and tolerate a wide range of water temperatures and [[salinity|salinities]].<ref name=PurcellShiganova2001MnemiopsisNativeAndExotic>{{cite journal |last1=Purcell |first1=Jennifer E. |last2=Shiganova |first2=Tamara A. |last3=Decker |first3=Mary Beth |last4=Houde |first4=Edward D. |year=2001 |title=The ctenophore ''Mnemiopsis'' in native and exotic habitats: U.S. estuaries versus the Black Sea basin |journal=Hydrobiologia |volume=451 |issue=1–3 |pages=145–176 |doi=10.1023/A:1011826618539 |bibcode=2001HyBio.451..145P |s2cid=23336715}}</ref> The impact was increased by chronic overfishing, and by [[eutrophication]] that gave the entire ecosystem a short-term boost, causing the ''Mnemiopsis'' population to increase even faster than normal<ref name=OguzFachSalihoglu2008InvasionDynamics>{{cite journal |last1=Oguz |first1=T. |last2=Fach |first2=B. |last3=Salihoglu |first3=B. |date=December 2008 |title=Invasion dynamics of the alien ctenophore ''Mnemiopsis leidyi'' and its impact on anchovy collapse in the Black Sea |journal=Journal of Plankton Research |volume=30 |issue=12 |pages=1385–1397 |doi=10.1093/plankt/fbn094 |doi-access=free|hdl=11511/30161 |hdl-access=free }}</ref> – and above all by the absence of efficient predators on these introduced ctenophores.<ref name="PurcellShiganova2001MnemiopsisNativeAndExotic"/> ''Mnemiopsis'' populations in those areas were eventually brought under control by the accidental introduction of the ''Mnemiopsis''-eating North American ctenophore ''[[Beroe ovata]]'',<ref>{{cite book |last1=Bayha |first1=K.M. |last2=Harbison |first2=R. |last3=McDonald |first3=J.H. |last4=Gaffney |first4=P.M. |year=2004 |section=Preliminary investigation on the molecular systematics of the invasive ctenophore ''Beroe ovata'' |editor1-last=Dumont |editor1-first=H. |editor2-last=Shiganova |editor2-first=T.A. |editor3-last=Niermann |editor3-first=U. |title=Aquatic invasions in the Black, Caspian, and Mediterranean seas |volume=35 |pages=167–175 |series=NATO Science Series&nbsp;IV: Earth and Environmental Sciences |publisher=Springer Netherlands |isbn=978-1-4020-1866-4 |doi=10.1007/1-4020-2152-6_7}}</ref> and by a cooling of the local climate from 1991 to 1993,<ref name=OguzFachSalihoglu2008InvasionDynamics/> which significantly slowed the animal's metabolism.<ref name=PurcellShiganova2001MnemiopsisNativeAndExotic/> However the abundance of plankton in the area seems unlikely to be restored to pre-''Mnemiopsis'' levels.<ref>{{cite book |last=Kamburska |first=L. |year=2006 |section=Effects of ''Beroe'' c.f. ''ovata'' on gelatinous and other zooplankton along the Bulgarian Black Sea Coast |editor1-last=Dumont |editor1-first=H. |editor2-last=Shiganova |editor2-first=T.A. |editor3-last=Niermann |editor3-first=U. |title=Aquatic Invasions in the Black, Caspian, and Mediterranean Seas |volume=35 |pages=137–154 |series=NATO Science Series&nbsp;IV: Earth and Environmental Sciences |publisher=Springer Netherlands |isbn=978-1-4020-1866-4 |doi=10.1007/1-4020-2152-6_5}}</ref>

In the late 1990s ''Mnemiopsis'' appeared in the [[Caspian Sea]].<!---probably via the canals that connect the Caspian to the Black Sea --- There are no salt water channels connecting the Caspian and the Black seas ---> ''Beroe ovata'' arrived shortly after, and is expected to reduce but not eliminate the impact of ''Mnemiopsis'' there. ''Mnemiopsis'' also reached the eastern [[Mediterranean]] in the late 1990s and now appears to be thriving in the [[North Sea]] and [[Baltic Sea]].<ref name=MillsNotesFromExpert/>

== Taxonomy ==

The number of known living ctenophore species is uncertain since many of those named and formally described have turned out to be identical to species known under other scientific names. Claudia Mills estimates that there about 100–150&nbsp;valid species that are not duplicates, and that at least another&nbsp;25, mostly deep-sea forms, have been recognized as distinct but not yet analyzed in enough detail to support a formal description and naming.<ref name=MillsCtenoList/>

=== Early classification ===

Early writers combined ctenophores with [[cnidarians]] into a single phylum called [[Coelenterata]] on account of morphological similarities between the two groups. Like cnidarians, the bodies of ctenophores consist of a mass of jelly, with one layer of [[cell (biology)|cells]] on the outside and another lining the internal cavity. In ctenophores, however, these layers are two cells deep, while those in cnidarians are only a single cell deep. Ctenophores also resemble cnidarians in relying on water flow through the body cavity for both digestion and respiration, as well as in having a decentralized [[nerve net]] rather than a brain.<ref name="Dunn2015">{{cite journal |last1=Dunn |first1=Casey W. |last2=Leys |first2=Sally P. |last3=Haddock |first3=Steven H.D. |author3-link=Steven Haddock |title=The hidden biology of sponges and ctenophores |journal=Trends in Ecology & Evolution |date=May 2015 |volume=30 |issue=5 |pages=282–291 |doi=10.1016/j.tree.2015.03.003 |doi-access=free |pmid=25840473 |bibcode=2015TEcoE..30..282D }}</ref> Genomic studies have suggested that the [[neuron]]s of Ctenophora, which differ in many ways from other animal neurons, evolved independently from those of the other animals.<ref>{{cite news |title=Comb jelly nseurons spark evolution debate |magazine=[[Quanta Magazine]] |date=2015-03-25 |url=https://www.quantamagazine.org/20150325-did-neurons-evolve-twice/ |access-date=2015-06-12}}</ref>

===Modern taxonomy===

[[File:Lobate ctenophore.jpg|thumb| [[Lobata]] sp., with paired thick lobes]]

The traditional classification divides ctenophores into two [[class (biology)|classes]], those with tentacles ([[Tentaculata]]) and those without ([[Nuda]]). The Nuda contains only one [[order (biology)|order]] ([[Beroida]]) and [[family (biology)|family]] ([[Beroidae]]), and two [[genus|genera]], ''Beroe'' (several species) and ''[[Neis]]'' (one species).<ref name=MillsCtenoList/>

The [[Tentaculata]] are divided into the following eight [[order (biology)|orders]]:<ref name=MillsCtenoList/>

* [[Cydippida]], egg-shaped animals with long tentacles<ref name=RuppertBarnes2004Ctenophora/>
* [[Lobata]], with paired thick lobes<ref name=RuppertBarnes2004Ctenophora/>
* [[Platyctenida]], flattened animals that live on or near the sea-bed; most lack combs as adults, and use their [[pharynx|pharynges]] as suckers to attach themselves to surfaces<ref name=RuppertBarnes2004Ctenophora/>
* [[Ganeshida]], with a pair of small lobes round the mouth, but an extended [[pharynx]] like that of platyctenids<ref name=RuppertBarnes2004Ctenophora/>
* [[Cambojiida]]
* [[Cryptolobiferida]]
* [[Thalassocalycida]], with short tentacles and a jellyfish-like "umbrella"<ref name=RuppertBarnes2004Ctenophora/>
* [[Cestida]], ribbon-shaped and the largest ctenophores<ref name=RuppertBarnes2004Ctenophora/>

==Evolutionary history==

Despite their fragile, gelatinous bodies, [[fossil]]s thought to represent ctenophores – apparently with no tentacles but many more comb-rows than modern forms – have been found in [[Lagerstätte]]n as far back as the early [[Cambrian]], about {{ma|515}}. Nevertheless, a recent molecular phylogenetics analysis concludes that the common ancestor originated approximately 350&nbsp;million years ago ± 88&nbsp;million years ago, conflicting with previous estimates which suggests it occurred {{ma|Paleogene}} after the [[Cretaceous–Paleogene extinction event]].<ref name=Whelan2017/>

The following cladogram shows hypothesized relationships among some of the more well-understood members of the Ctenophora [[stem group]], with the [[Dinomischus|Dinomischidae]] seen as a paraphyletic grade:<ref>{{cite journal|last1=Klug |first1=Christian |last2=Kerr |first2=Johanne |last3=Lee |first3=Michael S. Y. |last4=Cloutier |first4=Richard |year=2021 |title=A late‐surviving stem‐ctenophore from the Late Devonian of Miguasha (Canada) |journal=Scientific Reports |article-number=19039 |doi=10.1038/s41598-021-98362-5 |doi-access=free}}</ref>

{{clade
  |label1='''Ctenophora'''
  |sublabel1={{small|([[total group]])}}
  |1={{clade
    |1="Dinomischids"
    |2={{clade
      |1=''Daihuoides''
      |2={{clade
        |1=[[Scleroctenophora]]
        |2='''Ctenophora''' {{small|([[crown group]])}}
      }}
    }}
  }}
}}
===Fossil record===

{{Further| Ctenorhabdotus capulus| Fasciculus vesanus| Xanioascus canadensis| Archaeocydippida hunsrueckiana| Paleoctenophora brasseli}}

Because of their soft, gelatinous bodies, ctenophores are extremely rare as fossils, and fossils that have been interpreted as ctenophores have been found only in [[Lagerstätte]]n, places where the environment was exceptionally suited to the preservation of soft tissue. Until the mid-1990s, only two specimens good enough for analysis were known, both members of the crown group, from the early [[Devonian]] (Emsian) [[geologic timescale|period]]. Three additional putative species were then found in the [[Burgess Shale]] and other Canadian rocks of similar age, about {{ma|505}} in the mid-[[Cambrian]] period. All three lacked tentacles but had between 24–80&nbsp;comb rows, far more than the eight typical of living species. They also appear to have had internal organ-like structures unlike anything found in living ctenophores. One of the fossil species first reported in 1996 had a large mouth, apparently surrounded by a folded edge that may have been muscular.<ref name=Morris1996/> Evidence from China a year later suggests that such ctenophores were widespread in the Cambrian, but perhaps very different from modern species – for example one fossil's comb-rows were mounted on prominent vanes.<ref>{{cite journal |last=Conway Morris |first=S. |author-link=Simon Conway Morris |year=2003 |title=The Cambrian "explosion" of metazoans and molecular biology: Would Darwin be satisfied? |journal=[[International Journal of Developmental Biology]] |volume=47 |issue=7–8 |pages=505–515 |pmid=14756326 |url=http://www.ijdb.ehu.es/fullaccess/fulltext.03078/ft505.pdf |access-date=2009-02-14 |archive-url=https://web.archive.org/web/20091224184258/http://www.ijdb.ehu.es/fullaccess/fulltext.03078/ft505.pdf
 |archive-date=2009-12-24}}</ref> The youngest fossil of a species outside the crown group is ''[[Daihuoides]]'' from the late Devonian, which belongs to a basal group that had been assumed to have gone extinct more than 140 million years earlier.<ref>{{cite journal | last1=Klug | first1=Christian | last2=Kerr | first2=Johanne | last3=Lee | first3=Michael S.Y. | last4=Cloutier | first4=Richard | year=2021 | title=A late-surviving stem-ctenophore from the Late Devonian of Miguasha (Canada) | journal=[[Scientific Reports]] | volume=11 | issue=1 | page=19039 | pmid=34561497 | pmc=8463547 | bibcode=2021NatSR..1119039K | doi=10.1038/s41598-021-98362-5 }}</ref>

The Ediacaran ''[[Eoandromeda]]'' could putatively represent a comb jelly.<ref name=Tang2011>{{Cite journal | last1=Tang | first1=F. | last2=Bengtson | first2=S. | last3=Wang | first3=Y. | last4=Wang | first4=X.L. | last5=Yin | first5=C.Y. | date=20 September 2011 | title=Eoandromeda and the origin of Ctenophora | journal=[[Evolution & Development]] | volume=13 | issue=5 | pages=408–414 | doi=10.1111/j.1525-142X.2011.00499.x | pmid=23016902| s2cid=28369431 }}</ref> It has eightfold symmetry, with eight spiral arms resembling the comblike rows of a ctenophore. If it is indeed ctenophore, it places the group close to the origin of the Bilateria.<ref>{{cite news |last1=Maxmen |first1=Amy |date=7 September 2011 |title=Ancient sea jelly shakes evolutionary tree of animals |magazine=[[Scientific American]] |url=https://www.scientificamerican.com/article/ancient-sea-jelly-makes-tree/ |access-date=21 June 2018 }}</ref>
The early Cambrian [[Sessility (zoology)|sessile]] [[frond]]-like fossil ''[[Stromatoveris]]'', from China's [[Chengjiang fauna|Chengjiang]] lagerstätte and dated to about {{ma|515}}, is very similar to [[Vendobionta]] of the preceding [[Ediacaran]] period. De-Gan Shu, [[Simon Conway Morris]], ''et al.'' found on its branches what they considered rows of cilia, used for [[filter feeding]]. They suggested that ''Stromatoveris'' was an evolutionary "aunt" of ctenophores, and that ctenophores originated from sessile animals whose descendants became swimmers and changed the cilia from a feeding mechanism to a propulsion system.<ref>{{cite journal |last1=Shu |first1=D.-G. |last2=Conway Morris |author2-link=Simon Conway Morris |first2=S. |last3=Han |first3=J. |last4=Li |first4=Y. |last5=Zhang |first5=X.L. |last6=Hua |first6=H. |last7=Zhang |first7=Z.F. |last8=Liu |first8=JN |last9=Guo |first9=J.F. |last10=Yao |first10=Y. |last11=Yasui |first11=K. |display-authors=6 |title=Lower Cambrian vendobionts from China and early diploblast evolution |journal=[[Science (journal)|Science]] |date=5 May 2006 |volume=312 |issue=5774 |pages=731–734 |doi=10.1126/science.1124565 |pmid=16675697 |bibcode=2006Sci...312..731S |s2cid=1235914 }}</ref> Other Cambrian fossils that support the idea of ctenophores having evolved from sessile forms are ''[[Dinomischus]]'', ''[[Daihua]]'', ''[[Xianguangia]]'' and ''[[Siphusauctum]]'' which also lived on the seafloor, had organic skeletons and cilia-covered tentacles surrounding their mouth, which have been found by [[cladistics|cladistic]] analysis as members of the ctenophore [[stem-group]]<ref>{{cite journal |last1=Zhao |first1=Yang |last2=Vinther |first2=Jakob |last3=Parry |first3=Luke A. |last4=Wei |first4=Fan |last5=Green |first5=Emily |last6=Pisani |first6=Davide |last7=Hou |first7=Xianguang |last8=Edgecombe |first8=Gregory D. |last9=Cong |first9=Peiyun |display-authors=6 |date=April 2019 |title=Cambrian sessile, suspension feeding stem-group Ctenophores and evolution of the comb jelly body plan |journal=[[Current Biology]] |lang=en |volume=29 |issue=7 |pages=1112–1125.e2 |doi=10.1016/j.cub.2019.02.036 |doi-access=free |pmid=30905603 |hdl=1983/40a6bcb8-a740-482c-a23c-7d563faea5c5 |hdl-access=free }}</ref><ref>{{Cite journal |last1=Zhao |first1=Yang |last2=Hou |first2=Xian-guang |last3=Cong |first3=Pei-yun |date=January 2023 |title=Tentacular nature of the 'column' of the Cambrian diploblastic ''Xianguangia sinica'' |journal=[[Journal of Systematic Palaeontology]] |lang=en |volume=21 |issue=1 |article-number=2215787 |doi=10.1080/14772019.2023.2215787 |doi-access=free |bibcode=2023JSPal..2115787Z }}</ref>

520&nbsp;million-year-old Cambrian fossils also from Chengjiang in China show a now wholly extinct class of ctenophore, named "[[Scleroctenophora]]", that had a complex internal skeleton with long spines.<ref>{{cite web |last=Mindy |first=Weisberger |date=2015-07-10 |title=Ancient jellies had spiny skeletons, no tentacles |website=livescience.com |url=https://www.livescience.com/51515-ancient-comb-jellies-had-skeletons.html |lang=en}}</ref> The skeleton also supported eight soft-bodied flaps, which could have been used for swimming and possibly feeding. One form, ''[[Thaumactena]]'', had a streamlined body resembling that of [[Chaetognatha|arrow worms]] and could have been an agile swimmer.<ref name=Shu-Zhang-etal-2015/>

===Relationship to other animal groups===

The [[phylogenetic tree|phylogenetic]] relationship of ctenophores to the rest of [[Metazoa]] is very important to our understanding of the early evolution of animals and the origin of multicellularity. It has been the focus of debate for many years. Ctenophores have been purported to be the sister lineage to the [[Bilateria]],<ref name="Simonetta1991">{{cite book |editor1-first=Simon |editor1-last=Conway Morris |editor1-link=Simon Conway Morris |editor2-first=Alberto M. |editor2-last=Simonetta |year=1991 |title=The Early Evolution of Metazoa and the Significance of Problematic Taxa |publisher=Cambridge University Press |isbn=978-0-521-11158-4 |page=308 }}</ref><ref name=Nielsen1996>{{cite journal |last1=Nielsen |first1=Claus |last2=Scharff |first2=Nikolaj |last3=Eibye-Jacobsen |first3=Danny |date=April 1996 |title=Cladistic analyses of the animal kingdom |journal=[[Biological Journal of the Linnean Society]] |volume=57 |issue=4 |pages=385–410 |doi=10.1006/bijl.1996.0023 |doi-access=free|bibcode=1996BJLS...57..385N }}</ref> sister to the [[Cnidaria]],<ref name=Leuckart1848>{{cite book |last=Leuckart |first=Rudolf |year=1923 |title=Ueber die Morphologie und die Verwandtschaftsverhältnisse der wirbellosen thiere. Ein Beitrag zur Charakteristik und Classification der thierischen Formen |lang=de}}</ref><ref name=Haeckel1896>{{cite book |last=Haeckel |first=Ernst |author-link=Ernst Heinrich Philipp August Haeckel |year=1896 |title=Systematische Phylogenie der Wirbellosen Thiere |series=Invertebrata |volume=Part&nbsp;2: Des Entwurfs Einer Systematischen Stammesgeschichte |lang=de }}</ref><ref name=Hyman1940>{{cite book |last=Hyman |first=Libbie Henrietta |year=1940 |title=The Invertebrates |volume=I&nbsp;Protozoa through Ctenophora |publisher=McGraw Hill |isbn=978-0-07-031660-7}}</ref><ref name=Philippe2009>{{cite journal |last1=Philippe |first1=H. |last2=Derelle |first2=R. |last3=Lopez |first3=P. |last4=Pick |first4=K. |last5=Borchiellini |first5=C. |last6=Boury-Esnault |first6=N. |last7=Vacelet |first7=J. |last8=Renard |first8=E. |last9=Houliston |first9=E. |last10=Quéinnec |first10=E. |last11=Da Silva |first11=C. |last12=Wincker |first12=P. |last13=Le Guyader |first13=H. |last14=Leys |first14=S. |last15=Jackson |first15=D.J. |last16=Schreiber |first16=F. |last17=Erpenbeck |first17=D. |last18=Morgenstern |first18=B. |last19=Wörheide |first19=G. |last20=Manuel |first20=M.L. |display-authors=6 |date=28 April 2009 |title=Phylogenomics Revives Traditional Views on Deep Animal Relationships |journal=[[Current Biology]] |volume=19 |issue=8 |pages=706–712 |doi=10.1016/j.cub.2009.02.052 |doi-access=free |pmid=19345102 |s2cid=15282843 |bibcode=2009CBio...19..706P }}</ref> [[Placozoa]], and [[Bilateria]],<ref name=WallbergEtAl2004PhylogeneticPositionOfCtenophora>{{Cite journal |last1=Wallberg |first1=A. |last2=Thollesson |first2=M. |last3=Farris |first3=J.S. |last4=Jondelius |first4=U. |date=December 2004 |title=The phylogenetic position of the comb jellies (Ctenophora) and the importance of taxonomic sampling |journal=Cladistics |volume=20 |issue=6 |pages=558–578 |doi=10.1111/j.1096-0031.2004.00041.x |pmid=34892961 |s2cid=86185156|doi-access=free }}</ref><ref name=Collins2002PhylogenyOfMedusozoa>{{cite journal |last=Collins |first=A.G. |year=2002 |title=Phylogeny of Medusozoa and the evolution of cnidarian life cycles |journal=[[Journal of Evolutionary Biology]] |volume=15 |issue=3 |pages=418–432 |s2cid=11108911 |doi=10.1046/j.1420-9101.2002.00403.x |doi-access=free }}</ref><ref name=Pick2010>{{cite journal |last1=Pick |first1=K.S. |last2=Philippe |first2=H. |last3=Schreiber |first3=F. |last4=Erpenbeck |first4=D. |last5=Jackson |first5=D.J. |last6=Wrede |first6=P. |last7=Wiens |first7=M. |last8=Alié |first8=A. |last9=Morgenstern |first9=B. |last10=Manuel |first10=M. |last11=Wörheide |first11=G. |display-authors=6 |date=September 2010 |title=Improved phylogenomic taxon sampling noticeably affects non-bilaterian relationships |journal=[[Molecular Biology and Evolution]] |volume=27 |issue=9 |pages=1983–1987 |doi=10.1093/molbev/msq089 |pmc=2922619 |pmid=20378579 }}</ref> and sister to all other animals.<ref name=Dunn2008/><ref name=Hejnol2009>{{Cite journal |last1=Hejnol |first1=A. |last2=Obst |first2=M. |last3=Stamatakis |first3=A. |last4=Ott |first4=M. |last5=Rouse |first5=G.W. |last6=Edgecombe |first6=G.D. |last7=Martinez |first7=P. |last8=Baguna |first8=J. |last9=Bailly |first9=X. |last10=Jondelius |first10=U. |last11=Wiens |first11=M. |last12=Muller |first12=W.E.G. |last13=Seaver |first13=E. |last14=Wheeler |first14=W.C. |last15=Martindale |first15=M.Q. |last16=Giribet |first16=G. |last17=Dunn |first17=C. W. |display-authors=6 |date=22 December 2009 |title=Assessing the root of bilaterian animals with scalable phylogenomic methods |journal=Proceedings of the Royal Society B: Biological Sciences |volume=276 |issue=1677 |pages=4261–4270 |doi=10.1098/rspb.2009.0896 |pmc=2817096 |pmid=19759036}}</ref>

[[Walter Garstang]] in his book [[Walter Garstang#Larval Forms and Other Zoological Verses|''Larval Forms and Other Zoological Verses'']] (''Mülleria and the Ctenophore'') even expressed a theory that [[ctenophore]]s were descended from a [[neotenic]] ''[[Mülleria]]'' larva of a [[polyclad]].<ref name="Garstang 1985">{{cite book |last=Garstang |first=Walter |author-link=Walter Garstang |chapter=Mülleria and the Ctenophore |title=Larval Forms, and Other Zoological Verses |publisher=Heinemann Educational Publishers |publication-place=Chicago |date=1985 |orig-date=1951 |isbn=978-0-226-28423-1 |page=}}</ref>

A series of studies that looked at the presence and absence of members of gene families and signalling pathways (e.g., [[homeobox]]es, [[nuclear receptor]]s, the [[Wnt signaling pathway]], and [[sodium channels]]) suggest that ctenophores are either sister to [[Cnidaria]], [[Placozoa]], and [[Bilateria]] or sister to all other animal phyla.<ref name="Ryan2010">{{Cite journal |last1=Ryan |first1=J. F. |last2=Pang |first2=K. |last3=Comparative Sequencing Program |last4=Mullikin |first4=J. C. |last5=Martindale |first5=M. Q. |last6=Baxevanis |first6=A. D. |last7=NISC Comparative Sequencing Program |year=2010 |title=The homeodomain complement of the ctenophore ''Mnemiopsis leidyi'' suggests that Ctenophora and Porifera diverged prior to the ParaHoxozoa |journal=[[EvoDevo]] |volume=1 |issue=1 |page=9 |doi=10.1186/2041-9139-1-9 |pmc=2959044 |pmid=20920347 |doi-access=free }}</ref><ref name="Reitzel2011">{{Cite journal |last1=Reitzel |first1=A. M. |last2=Pang |first2=K. |last3=Ryan |first3=J. F. |last4=Mullikin |first4=J. C. |last5=Martindale |first5=M. Q. |last6=Baxevanis |first6=A. D. |last7=Tarrant |first7=A. M. |year=2011 |title=Nuclear receptors from the ctenophore ''Mnemiopsis leidyi'' lack a zinc-finger DNA-binding domain: Lineage-specific loss or ancestral condition in the emergence of the nuclear receptor superfamily? |journal=[[EvoDevo]] |volume=2 |issue=1 |page=3 |doi=10.1186/2041-9139-2-3 |pmc=3038971 |pmid=21291545 |doi-access=free }}</ref><ref name="Pang2011">{{Cite journal |last1=Pang |first1=K. |last2=Ryan |first2=J. F. |last3=NISC Comparative Sequencing Program |first3=<!-- comment to keep out citation bot --> |last4=Mullikin |first4=J. C. |last5=Baxevanis |first5=A. D. |last6=Martindale |first6=M. Q. |year=2010 |title=Genomic insights into Wnt signaling in an early diverging metazoan, the ctenophore ''Mnemiopsis leidyi'' |journal=[[EvoDevo]] |volume=1 |issue=1 |page=10 |doi=10.1186/2041-9139-1-10 |pmc=2959043 |pmid=20920349 |doi-access=free }}</ref><ref name="Liebeskind2011">{{Cite journal |last1=Liebeskind |first1=B. J. |last2=Hillis |first2=D. M. |last3=Zakon |first3=H. H. |year=2011 |title=Evolution of sodium channels predates the origin of nervous systems in animals |journal=Proceedings of the National Academy of Sciences of the USA |volume=108 |issue=22 |pages=9154–9159 |bibcode=2011PNAS..108.9154L |doi=10.1073/pnas.1106363108 |pmc=3107268 |pmid=21576472 |doi-access=free}}</ref> 
Several more recent studies comparing complete sequenced genomes of ctenophores with other sequenced animal genomes support ctenophores as sister to all other animals.<ref name=Ryan2013>{{Cite journal |last1=Ryan |first1=J.F. |last2=Pang |first2=K. |last3=Schnitzler |first3=C.E. |last4=Nguyen |first4=A.-D. |last5=Moreland |first5=R.T. |last6=Simmons |first6=D.K. |last7=Koch |first7=B.J. |last8=Francis |first8=W.R. |last9=Havlak |first9=P. |last10=Smith |first10=S.A. |last11=Putnam |first11=N.H. |last12=Haddock |first12=S.H.D. |last13=Dunn |first13=C.W. |last14=Wolfsberg |first14=T.G. |last15=Mullikin |first15=J.C. |last16=Martindale |first16=M.Q. |last17=Baxevanis |first17=A.D. |display-authors=6 |date=13 December 2013 |title=The Genome of the Ctenophore ''Mnemiopsis leidyi'' and its Implications for Cell Type Evolution |journal=[[Science (journal)|Science]] |volume=342 |number=6164 |article-number=1242592 |doi=10.1126/science.1242592 |pmc=3920664 |pmid=24337300}}</ref><ref name=Moroz2014/><ref>{{cite journal |last1=Whelan |first1=Nathan V. |last2=Kocot |first2=Kevin M. |last3=Moroz |first3=Leonid L. |last4=Halanych |first4=Kenneth M. |date=5 May 2015 |title=Error, signal, and the placement of Ctenophora sister to all other animals |journal=Proceedings of the National Academy of Sciences of the USA |volume=112 |issue=18 |pages=5773–5778 |bibcode=2015PNAS..112.5773W |doi=10.1073/pnas.1503453112 |pmc=4426464 |pmid=25902535 |doi-access=free}}</ref><ref>{{cite journal |last1=Borowiec |first1=Marek L. |last2=Lee |first2=Ernest K. |last3=Chiu |first3=Joanna C. |last4=Plachetzki |first4=David C. |date=December 2015 |title=Extracting phylogenetic signal and accounting for bias in whole-genome data sets supports the Ctenophora as sister to remaining Metazoa |journal=BMC Genomics |volume=16 |issue=1 |page=987 |doi=10.1186/s12864-015-2146-4 |pmc=4657218 |pmid=26596625 |doi-access=free}}</ref> This position would suggest that neural and muscle cell types either were lost in major animal lineages (e.g., [[Porifera]] and [[Placozoa]]) or evolved independently in the ctenophore lineage.<ref name=Ryan2013/>

Other researchers have argued that the placement of Ctenophora as sister to all other animals is a statistical anomaly caused by the high rate of evolution in ctenophore genomes, and that [[Porifera]] is the earliest-diverging animal taxon instead (a "sponge sister" topology).<ref name="Pick2010" /><ref>{{cite journal |last1=Philippe |first1=Hervé |last2=Derelle |first2=Romain |last3=Lopez |first3=Philippe |last4=Pick |first4=Kerstin |last5=Borchiellini |first5=Carole |last6=Boury-Esnault |first6=Nicole |last7=Vacelet |first7=Jean |last8=Renard |first8=Emmanuelle |last9=Houliston |first9=Evelyn |last10=Quéinnec |first10=Eric |last11=da Silva |first11=Corinne |last12=Wincker |first12=Patrick |last13=Le Guyader |first13=Hervé |last14=Leys |first14=Sally |last15=Jackson |first15=Daniel J. |last16=Schreiber |first16=Fabian |last17=Erpenbeck |first17=Dirk |last18=Morgenstern |first18=Burkhard |last19=Wörheide |first19=Gert |last20=Manuel |first20=Michaël |display-authors=6 |date=April 2009 |title=Phylogenomics revives traditional views on deep animal relationships |journal=[[Current Biology]] |volume=19 |issue=8 |pages=706–712 |doi=10.1016/j.cub.2009.02.052 |doi-access=free |pmid=19345102 |s2cid=15282843 |bibcode=2009CBio...19..706P}}</ref><ref>{{cite journal |last1=Nosenko |first1=Tetyana |last2=Schreiber |first2=Fabian |last3=Adamska |first3=Maja |last4=Adamski |first4=Marcin |last5=Eitel |first5=Michael |last6=Hammel |first6=Jörg |last7=Maldonado |first7=Manuel |last8=Müller |first8=Werner E.G. |last9=Nickel |first9=Michael |display-authors=6 |date=April 2013 |title=Deep metazoan phylogeny: When different genes tell different stories |journal=[[Molecular Phylogenetics and Evolution]] |volume=67 |issue=1 |pages=223–233 |doi=10.1016/j.ympev.2013.01.010 |pmid=23353073 |bibcode=2013MolPE..67..223N |hdl=10261/93095 |hdl-access=free }}</ref><ref>{{cite journal |last1=Pisani |first1=Davide |last2=Pett |first2=Walker |last3=Dohrmann |first3=Martin |last4=Feuda |first4=Roberto |last5=Rota-Stabelli |first5=Omar |last6=Philippe |first6=Hervé |last7=Lartillot |first7=Nicolas |last8=Wörheide |first8=Gert |display-authors=6 |date=15 December 2015 |title=Genomic data do not support comb jellies as the sister group to all other animals |journal=Proceedings of the National Academy of Sciences of the USA |volume=112 |issue=50 |pages=15402–15407 |bibcode=2015PNAS..11215402P |doi-access=free |doi=10.1073/pnas.1518127112 |pmc=4687580 |pmid=26621703}}</ref><ref name=Kapli_2020>{{cite journal |last1=Kapli |first1=Paschalia |last2=Telford |first2=Maximilian J. |date=11 December 2020 |title=Topology-dependent asymmetry in systematic errors affects phylogenetic placement of Ctenophora and Xenacoelomorpha |journal=[[Science Advances]] |volume=6 |issue=10 |article-number=eabc5162 |bibcode=2020SciA....6.5162K |pmc=7732190 |doi=10.1126/sciadv.abc5162 |pmid=33310849}}</ref>They also have extremely high rates of [[Mitochondrion|mitochondrial]] evolution,<ref>{{cite journal | last1=Christianson | first1=L.M. | last2=Johnson | first2=S.B. | last3=Schultz | first3=D.T. | last4=Haddock | first4=S.H. | year=2021 | title=Hidden diversity of Ctenophora revealed by new mitochondrial COI primers and sequences | journal=[[Molecular Ecology Resources]] | volume=22 | issue=1 | pages=283–294 | doi=10.1111/1755-0998.13459 | pmid=34224654 | pmc=9290464}}</ref>and the smallest known RNA/protein content of the [[Mitochondrial DNA|mtDNA genome]] in animals.<ref>{{cite journal | last1=Kohn | first1=A.B. | last2=Citarella | first2=M.R. | last3=Kocot | first3=K.M. | last4=Bobkova | first4=Y.V. | last5=Halanych | first5=K.M. | last6=Moroz | first6=L.L. | year=2011 | title=Rapid evolution of the compact and unusual mitochondrial genome in the ctenophore, ''Pleurobrachia bachei'' | journal=[[Molecular Phylogenetics and Evolution]] | volume=63 | issue=1 | pages=203–207 | doi=10.1016/j.ympev.2011.12.009 | pmc=4024468 | pmid=22201557}}</ref> As such, the Ctenophora appear to be a basal [[diploblast]] clade. In agreement with the latter point, the analysis of a very large sequence alignment at the metazoan taxonomic scale (1,719&nbsp;proteins totalizing {{nobr|{{circa|{{gaps|400 000}} amino}}}} acid positions) in Simion ''et al.'' (2017) showed that ctenophores emerge as the second-earliest branching animal lineage, and sponges are sister to all other multicellular animals.<ref name=Simion2017/> Research on [[mucin]] genes shows that sponges have never had them while all other animals, including comb jellies, do.<ref>{{cite journal |last1=Bakshani |first1=Cassie R. |last2=Morales-Garcia |first2=Ana L. |last3=Althaus |first3=Mike |last4=Wilcox |first4=Matthew D. |last5=Pearson |first5=Jeffrey P. |last6=Bythell |first6=John C. |last7=Burgess |first7=J. Grant |date=4 July 2018 |title=Evolutionary conservation of the antimicrobial function of mucus: A first defence against infection |journal=[[npj Biofilms and Microbiomes]] |volume=4 |issue=1 |page=14 |doi=10.1038/s41522-018-0057-2 |pmc=6031612 |pmid=30002868}}</ref>

Despite all their differences, ctenophoran neurons share the same foundation as cnidarian neurons after findings shows that peptide-expressing neurons are probably ancestral to chemical neurotransmitters.<ref>{{cite journal |first1 =Eisuke |last1 =Hayakawa |first2 =Christine |last2 =Guzman |first3 =Osamu |last3 =Horiguchi |first4 =Chihiro |last4 =Kawano |first5 =Kurato |last5 =Mohri |first6 =Mei-Fang |last6 =Lin |first7 =Ryotaro |last7 =Nakamura |first8 =Erina |last8 =Kawai |first9 =Shinya |last9 =Komoto |first10=Kogiku |last10=Shiba |first11=Shuji |last11=Shigenobu |first12=Honoo |last12=Satake |first13=Kazuo |last13=Inaba |first14=Hiroshi |last14=Watanabe |display-authors=6 |date=October 2022 |orig-date=8 August 2022 |title=Mass spectrometry of short peptides reveals common features of metazoan peptidergic neurons |journal=[[Nature Ecology and Evolution]] |volume=6 |issue= 10|pages=1438–1448 |doi=10.1038/s41559-022-01835-7 |doi-access=free |pmid =35941202 |pmc =9525235 |bibcode =2022NatEE...6.1438H }}</ref>

The issue with the "rate of evolution" counterargument is that it mainly affects analyses based on the sequence of genes, not those based on gene family presence or synteny, both of which have produced results in support of the "Ctenophora sister" theory.<ref name=Schultz23/> Even with sequence-based analyses, the biases can also be corrected for: Whelan ''et al.'' (2017), using such an approach, strongly rejects the hypothesis that sponges are the sister group to all other extant animals and establishes the placement of Ctenophora as the sister group to all other animals, and disagreement with Simion ''et al.'' (2017) is explained by methodological problems in analyses in that work.<ref name="Whelan2017"/> [[Synteny]] analysis by Schultz ''et al.'' (2023) supports the same result.<ref name=Schultz23/>
Neither ctenophores nor [[Porifera|sponges]] possess [[Hypoxia-inducible factors|HIF pathways]],<ref>{{Cite journal |last1=Mills |first1=DB |last2=Francis |first2=WR |last3=Vargas |first3=S |last4=Larsen |first4=M |last5=Elemans |first5=CP |last6=Canfield |first6=DE |last7=Wörheide |first7=G |year=2018 |title=The last common ancestor of animals lacked the HIF pathway and respired in low-oxygen environments |journal=eLife |volume=7 |article-number=e31176 |doi=10.7554/eLife.31176 |pmc=5800844 |pmid=29402379 |doi-access=free }}</ref> their genome express only a single type of [[voltage-gated calcium channel]] unlike other animals which have three types,<ref>{{Cite journal |last1=Gauberg |first1=Julia |last2=Abdallah |first2=Salsabil |last3=Elkhatib |first3=Wassim |last4=Harracksingh |first4=Alicia N. |last5=Piekut |first5=Thomas |last6=Stanley |first6=Elise F. |last7=Senatore |first7=Adriano |date=2020-12-25 |title=Conserved biophysical features of the CaV2 presynaptic Ca2+ channel homologue from the early-diverging animal Trichoplax adhaerens |journal=The Journal of Biological Chemistry |volume=295 |issue=52 |pages=18553–18578 |doi=10.1074/jbc.RA120.015725 |doi-access=free |pmc=7939481 |pmid=33097592}}</ref> and they are the only known animal phyla that lack any true [[Hox gene]]s.<ref name=Moroz2014/> A few species from other phyla; the [[nemertea]]n pilidium larva, the larva of the [[phoronid]] species ''Phoronopsis harmeri'' and the [[acorn worm]] larva ''Schizocardium californicum'', do not depend on Hox genes in their larval development either, but need them during metamorphosis to reach their adult form.<ref>{{cite report |first=Allan |last=Spradling |date=14 May 2012 |section=Relationships of early metazoans |title=Evolution and Development |page=38 |type=presentation slides |series=Evo-Devo research program |publisher=[[Carnegie Institution for Science|Carnagie Science]] |place=Washington, DC |url=http://emb.carnegiescience.edu/sites/emb.carnegiescience.edu/files/evodevo12.pdf |via=carnegiescience.edu |access-date=8 October 2024 |archive-url=https://web.archive.org/web/20140302084415/http://emb.carnegiescience.edu/sites/emb.carnegiescience.edu/files/evodevo12.pdf |archive-date=2014-03-02 }} — technical introduction and review of genomic research, evolution, and taxonomy </ref><ref>{{cite journal | last1=Hiebert | first1=Laurel S. | last2=Maslakova | first2=Svetlana A. | year=2015 | title=Hox genes pattern the anterior-posterior axis of the juvenile but not the larva in a maximally indirect developing invertebrate, Micrura alaskensis (Nemertea) | journal=[[BMC Biology]] | volume=13 | article-number=23 | pmid=25888821 | pmc=4426647 | doi=10.1186/s12915-015-0133-5 | doi-access=free }}</ref><ref>{{Cite journal |last1=Gąsiorowski |first1=Ludwik |last2=Hejnol |first2=Andreas |year=2019 |title=Hox gene expression during the development of the phoronid Phoronopsis harmeri |journal=[[EvoDevo]] |volume=11 |article-number=2 |biorxiv=10.1101/799056 |doi=10.1186/s13227-020-0148-z |pmc=7011278 |pmid=32064072 |s2cid=208578827 |doi-access=free }}</ref> [[Innexin]] genes, which code for proteins used for [[intercellular communication]] in animals, also appears to have evolved independently in ctenophores.<ref>[https://academic.oup.com/mbe/article/40/2/msad025/7026321?login=false Independent Innexin Radiation Shaped Signaling in Ctenophores]</ref>

===Internal phylogeny ===

<div style="float:right; width:auto; border:solid 1px silver; padding:2px; margin:2px; font-size:90%">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
|1={{clade
 |1=[[Mertensiidae]] ([[Cydippida|cydippid]]s)
 |2={{clade
 |1=[[Platyctenida]]
 |2={{clade
 |1=[[Pleurobrachiidae]] (cydippids)
 |2={{clade
 |1=[[Lobata]]
 |2=[[Thalassocalycida]]
 |3=[[Cestida]]
 }}
 |3={{clade
 |1=[[Haeckeliidae]] (cydippids)
 |2=[[Beroida]]
 }}
 }}
 }}
 }}
}}
</div>Relationships within Ctenophora (2001).<ref name="PodarHaddockEtAl2001MolecularPhylogeneticFrameworkForCtenophora" /></div>

<div style="float:right; width:auto; border:solid 1px silver; padding:2px; margin:2px; font-size:90%">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
|1={{clade
 |1=''[[Euplokamis]]''
 |2={{clade
 |1=[[Platyctenida]]
 |2={{clade
 |1=[[Pleurobrachiidae]]
 |2={{clade
 |1=[[Mertensiidae]]
 |2={{clade
 |1={{clade
 |1=[[Dryodoridae]]
 |2=[[Beroida]]
 }}
 |2=[[Lobata]] <small>incl. [[Cestida]]</small>
 }}
 }}
 }}
 }}
 }}
}}
</div>Relationships within Ctenophora (2017).<ref name=Whelan2017/></div>

Since all modern ctenophores except the beroids have cydippid-like larvae, it has widely been assumed that their last common ancestor also resembled cydippids, having an egg-shaped body and a pair of retractable tentacles. Richard Harbison's purely morphological analysis in 1985 concluded that the cydippids are not monophyletic, in other words do not contain all and only the descendants of a single common ancestor that was itself a cydippid. Instead, he found that various cydippid [[family (biology)|families]] were more similar to members of other ctenophore [[order (biology)|orders]] than to other cydippids. He suggested that the last common ancestor of modern ctenophores was either cydippid-like or beroid-like.<ref>{{cite book|last=Harbison|first=G.R.|title=The Origins and Relationships of Lower Invertebrates|url=https://archive.org/details/originsrelations00morr|url-access=limited |editor=Conway Morris, S. |editor-link=Simon Conway Morris |editor2=George, J.D. |editor3=Gibson, R. |editor4=Platt, H.M.|publisher=Clarendon Press|year=1985 |pages=[https://archive.org/details/originsrelations00morr/page/n47 78]–100|chapter=On the classification and evolution of the Ctenophora|isbn=978-0-19-857181-0}}</ref> 

A molecular phylogeny analysis in 2001, using 26 species, including four recently discovered ones, confirmed that the cydippids are not monophyletic and concluded that the last common ancestor of modern ctenophores was cydippid-like. It also found that the genetic differences between these species were so small that the relationships between the Lobata, Cestida and Thalassocalycida remained uncertain. This suggests that the last common ancestor of modern ctenophores was relatively recent, and perhaps survived the [[Cretaceous–Paleogene extinction event]] {{ma|65.5}} while other lineages perished. When the analysis was broadened to include representatives of other phyla, it concluded that cnidarians are probably more closely related to bilaterians than either group is to ctenophores but that this diagnosis is uncertain.<ref name=PodarHaddockEtAl2001MolecularPhylogeneticFrameworkForCtenophora>{{cite journal |last1=Podar |first1=Mircea |last2=Haddock |first2=Steven H.D. |last3=Sogin |first3=Mitchell L. |last4=Harbison |first4=G. Richard |date=November 2001 |title=A molecular phylogenetic framework for the phylum Ctenophora using 18S rRNA genes |journal=[[Molecular Phylogenetics and Evolution]] |volume=21 |issue=2 |pages=218–230 |doi=10.1006/mpev.2001.1036 |pmid=11697917 |bibcode=2001MolPE..21..218P |citeseerx=10.1.1.384.6705 }}</ref> A  2017 study corroborates the paraphyly of [[Cydippida]] but finds that [[Lobata]] is paraphyletic with respect to [[Cestida]].<ref name=Whelan2017/>

==See also==

* [[Gelatinous zooplankton]]

==References==

{{reflist|25em}}

== Further reading ==

{{refbegin|25em|small=yes}}
* {{cite book
 |first1=R.S.K. |last1=Barnes |first2=P. |last2=Calow
 |first3=P.J.W. |last3=Olive |first4=D.W. |last4=Golding
 |first5=J.I. |last5=Spicer
 |title=The Invertebrates: A synthesis |edition=3rd
 |publisher=Blackwell
 |year=2001
 |at=§&nbsp;3.4.3, p.&nbsp;63
 |isbn=0-632-04761-5 |ref=none
}}
* {{cite book
 |first1=R.C. |last1=Brusca
 |first2=G.J. |last2=Brusca
 |title=Invertebrates |edition=2nd
 |publisher=Sinauer Associates
 |year=2003
 |at=ch.&nbsp;9, p.&nbsp;269
 |isbn=0-87893-097-3 |ref=none
}}
* {{cite book
 |first=J. |last=Moore
 |title=An Introduction to the Invertebrates
 |publisher=Cambridge University Press
 |year=2001
 |at=§&nbsp;5.4, p.&nbsp;65
 |isbn=0-521-77914-6 |ref=none
}}
* {{cite book
 |first=W. |last=Schäfer
 |section=Ctenophora, Rippenquallen
 |editor1-first=W. |editor1-last=Westheide
 |editor2-first=R. |editor2-last=Rieger
 |title=Spezielle Zoologie |volume=1
 |publisher=Gustav Fischer Verlag
 |place=Stuttgart, DE
 |year=1996 |ref=none
}}
* {{cite book
 |first=Bruno |last=Wenzel
 |title=Glastiere des Meeres. Rippenquallen (Acnidaria)
 |year=1958
|ref=none
}}
* {{cite book
 |first=Mark |last=Shasha
 |title=Night of the Moonjellies
 |year=1992
 |publisher=Simon & Schuster
 |isbn=0-671-77565-0 |ref=none
}}
* {{cite magazine
 |first=Douglas |last=Fox
 |date=August 2017
 |title=Aliens in our midst: What the ctenophore says about the evolution of intelligence
 |magazine=[[Aeon (magazine)|Aeon]]
 |place=London, UK / New York, NY / Melbourne, AU
 |publisher=Aeon Media Group
 |url=https://aeon.co/essays/what-the-ctenophore-says-about-the-evolution-of-intelligence
 |via=Aeon.co
 |quote=The ctenophore's brain suggests that, if evolution began again, intelligence would re-emerge because nature repeats itself. |ref=none
}} — in part a review of research up to {{harvp|Moroz|Kocot|Citarella|Dosung|Norekian|Povolotskaya|Grigorenko|2014}} 
{{refend}}

== External links ==
{{sister project links|d=Q102778|c=category:Ctenophora|species=Ctenophora|q=no|v=no|voy=no|mw=no|m=no|s=no|n=no|b=no}}
* {{cite AV media
 |title=Plankton chronicles
 |medium=video
 |url=http://www.planktonchronicles.org/en/episode/14
 |archive-url=https://web.archive.org/web/20120112073501/http://www.planktonchronicles.org/en/episode/14
 |archive-date=2012-01-12
}} — Short documentary films & photos
* {{cite web
 |title=Jellyfish and comb jellies
 |date=30 April 2018
 |publisher=[[Smithsonian Institution]]
 |url=http://ocean.si.edu/jellyfish-and-comb-jellies
 |via=ocean.si.edu
}} — overview at the Smithsonian [http://ocean.si.edu/ Ocean portal ]
* {{cite web
 |title=Ctenophores from the São Sebastião Channel, Brazil
 |url=http://www.usp.br/cbm/artigos/ctenophora/index.htm
 |via=usp.br
 |archive-url=https://web.archive.org/web/20120204164759/http://www.usp.br/cbm/artigos/ctenophora/index.htm
 |archive-date=2012-02-04
}}
* {{cite press release
 |title=Tree of animal life has branches rearranged by evolutionary biologists
 |date=March 2008
 |website=sciencedaily.com
 |url=https://www.sciencedaily.com/releases/2008/03/080305144221.htm
}}
* {{cite report
 |title=Ctenophora
 |type=fact sheet
 |date=June 2008
 |department=Guide to the marine zooplankton of south eastern Australia
 |series=Tasmanian Aquaculture & Fisheries Institute / Australian Antarctic Division
 |publisher=Australian Government Department of the Environment, Water, Heritage, and the Arts
 |url=http://www.tafi.org.au/zooplankton/imagekey/ctenophora/index.html
 |via=tafi.org.au |url-status=usurped |access-date=2024-10-08
 |archive-url=https://web.archive.org/web/20080720150947/http://www.tafi.org.au/zooplankton/imagekey/ctenophora/index.html
 |archive-date=2008-07-20
}} — includes pictures
* {{cite AV media
 |medium=mixed video & still images
 |title=The jelly connection
 |website=imagequest3d.com
 |url=http://www.imagequest3d.com/pages/articles/jelly.htm
}} — striking images, including a ''Beroe'' specimen attacking another ctenophore
* {{cite magazine
 |title=In search of the first animals
 |date=2013-12-12
 |magazine=[[National Geographic (magazine)|National Geographic]]
 |url=http://phenomena.nationalgeographic.com/2013/12/12/in-search-of-the-first-animals/
 |archive-url=https://web.archive.org/web/20131212205447/http://phenomena.nationalgeographic.com/2013/12/12/in-search-of-the-first-animals/
 |archive-date=2013-12-12
}}

{{Animalia}}
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[[Category:Ctenophores| ]]
[[Category:Bioluminescent ctenophores]]
[[Category:Articles containing video clips]]
[[Category:Animal phyla]]
[[Category:Cambrian Series 2 first appearances]]
[[Category:Extant Cambrian first appearances]]